draft initial CONTRIBUTING guide (#567)

This guide was adopted from the node.js project.

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+# Contributing to Tokio
+
+:balloon: Thanks for your help improving the project! We are so happy to have
+you!
+
+There are opportunities to contribute to Tokio at any level. It doesn't matter if
+you are just getting started with Rust or are the most weathered expert, we can
+use your help.
+
+**No contribution is too small and all contributions are valued.**
+
+This guide will help you get started. **Do not let this guide intimidate you**.
+It should be considered a map to help you navigate the process.
+
+You may also find get help contributing in the [dev channel][dev], please join
+us!
+
+[dev]: https://gitter.im/tokio-rs/dev
+
+## Conduct
+
+The Tokio project adheres to the [Rust Code of Conduct][coc]. This describes
+the _minimum_ behavior expected from all contributors.
+
+[coc]: https://github.com/rust-lang/rust/blob/master/CODE_OF_CONDUCT.md
+
+## Contributing in Issues
+
+For any issue, there are fundamentally three ways an individual can contribute:
+
+1. By opening the issue for discussion: For instance, if you believe that you
+   have uncovered a bug in Tokio, creating a new issue in the tokio-rs/tokio
+   issue tracker is the way to report it.
+
+2. By helping to triage the issue: This can be done by providing
+   supporting details (a test case that demonstrates a bug), providing
+   suggestions on how to address the issue, or ensuring that the issue is tagged
+   correctly.
+
+3. By helping to resolve the issue: Typically this is done either in the form of
+   demonstrating that the issue reported is not a problem after all, or more
+   often, by opening a Pull Request that changes some bit of something in
+   Tokio in a concrete and reviewable manner.
+
+**Anybody can participate in any stage of contribution**. We urge you to
+participate in the discussion around bugs and participate in reviewing PRs.
+
+### Asking for General Help
+
+If you have reviewed existing documentation and still have questions or are
+having problems, you can open an issue asking for help.
+
+In exchange for receiving help, we ask that you contribute back a documentation
+PR that helps others avoid the problems that you encountered.
+
+### Submitting a Bug Report
+
+When opening a new issue in the Tokio issue tracker, users will be presented
+with a [basic template][template] that should be filled in. If you believe that you have
+uncovered a bug, please fill out this form, following the template to the best
+of your ability. Do not worry if you cannot answer every detail, just fill in
+what you can.
+
+The two most important pieces of information we need in order to properly
+evaluate the report is a description of the behavior you are seeing and a simple
+test case we can use to recreate the problem on our own. If we cannot recreate
+the issue, it becomes impossible for us to fix.
+
+In order to rule out the possibility of bugs introduced by userland code, test
+cases should be limited, as much as possible, to using only Tokio APIs.
+
+See [How to create a Minimal, Complete, and Verifiable example][mcve].
+
+[mcve]: https://stackoverflow.com/help/mcve
+[template]: .github/PULL_REQUEST_TEMPLATE.md
+
+### Triaging a Bug Report
+
+Once an issue has been opened, it is not uncommon for there to be discussion
+around it. Some contributors may have differing opinions about the issue,
+including whether the behavior being seen is a bug or a feature. This discussion
+is part of the process and should be kept focused, helpful, and professional.
+
+Short, clipped responses—that provide neither additional context nor supporting
+detail—are not helpful or professional. To many, such responses are simply
+annoying and unfriendly.
+
+Contributors are encouraged to help one another make forward progress as much as
+possible, empowering one another to solve issues collaboratively. If you choose
+to comment on an issue that you feel either is not a problem that needs to be
+fixed, or if you encounter information in an issue that you feel is incorrect,
+explain why you feel that way with additional supporting context, and be willing
+to be convinced that you may be wrong. By doing so, we can often reach the
+correct outcome much faster.
+
+### Resolving a Bug Report
+
+In the majority of cases, issues are resolved by opening a Pull Request. The
+process for opening and reviewing a Pull Request is similar to that of opening
+and triaging issues, but carries with it a necessary review and approval
+workflow that ensures that the proposed changes meet the minimal quality and
+functional guidelines of the Tokio project.
+
+## Pull Requests
+
+Pull Requests are the way concrete changes are made to the code, documentation,
+and dependencies in the Tokio repository.
+
+Even tiny pull requests (e.g., one character pull request fixing a typo in API
+documentation) are greatly appreciated. Before making a large change, it is
+usually a good idea to first open an issue describing the change to solicit
+feedback and guidance. This will increasethe likelihood of the PR getting
+merged.
+
+### Commits
+
+It is a recommended best practice to keep your changes as logically grouped as
+possible within individual commits. There is no limit to the number of commits
+any single Pull Request may have, and many contributors find it easier to review
+changes that are split across multiple commits.
+
+That said, if you have a number of commits that are "checkpoints" and don't
+represent a single logical change, please squash those together.
+
+Note that multiple commits often get squashed when they are landed (see the
+notes about [commit squashing]).
+
+#### Commit message guidelines
+
+A good commit message should describe what changed and why.
+
+1. The first line should:
+
+  * contain a short description of the change (preferably 50 characters or less,
+    and no more than 72 characters)
+  * be entirely in lowercase with the exception of proper nouns, acronyms, and
+    the words that refer to code, like function/variable names
+  * be prefixed with the name of the sub crate being changed (without the `tokio-`
+    prefix) and start with an imperative verb. If modifying `tokio` proper,
+    omit the crate prefix.
+
+  Examples:
+
+  * timer: introduce `Timeout` and deprecate `Deadline`
+  * export `Encoder`, `Decoder`, `Framed*` from tokio_codec
+
+2. Keep the second line blank.
+3. Wrap all other lines at 72 columns (except for long URLs).
+4. If your patch fixes an open issue, you can add a reference to it at the end
+   of the log. Use the `Fixes: #` prefix and the issue number. For other
+   references use `Refs: #`. `Refs` may include multiple issues, separated by a
+   comma.
+
+   Examples:
+
+   - `Fixes: #1337`
+   - `Refs: #1234`
+
+Sample complete commit message:
+
+```txt
+subcrate: explain the commit in one line
+
+Body of commit message is a few lines of text, explaining things
+in more detail, possibly giving some background about the issue
+being fixed, etc.
+
+The body of the commit message can be several paragraphs, and
+please do proper word-wrap and keep columns shorter than about
+72 characters or so. That way, `git log` will show things
+nicely even when it is indented.
+
+Fixes: #1337
+Refs: #453, #154
+```
+
+### Opening the Pull Request
+
+From within GitHub, opening a new Pull Request will present you with a
+[template] that should be filled out. Please try to do your best at filling out
+the details, but feel free to skip parts if you're not sure what to put.
+
+[template]: .github/PULL_REQUEST_TEMPLATE.md
+
+### Discuss and update
+
+You will probably get feedback or requests for changes to your Pull Request.
+This is a big part of the submission process so don't be discouraged! Some
+contributors may sign off on the Pull Request right away, others may have
+more detailed comments or feedback. This is a necessary part of the process
+in order to evaluate whether the changes are correct and necessary.
+
+**Any community member can review a PR and you might get conflicting feedback**.
+Keep an eye out for comments from code owners to provide guidance on conflicting
+feedback.
+
+**Once the PR is open, do not rebase the commits**. See [Commit Squashing] for
+more details.
+
+### Commit Squashing
+
+In most cases, **do not squash commits that you add to your Pull Request during
+the review process**. When the commits in your Pull Request land, they may be
+squashed into one commit per logical change. Metadata will be added to the
+commit message (including links to the Pull Request, links to relevant issues,
+and the names of the reviewers). The commit history of your Pull Request,
+however, will stay intact on the Pull Request page.
+
+## Reviewing Pull Requests
+
+**Any Tokio community member is welcome to review any pull request**.
+
+All Tokio contributors who choose to review and provide feedback on Pull
+Requests have a responsibility to both the project and the individual making the
+contribution. Reviews and feedback must be helpful, insightful, and geared
+towards improving the contribution as opposed to simply blocking it. If there
+are reasons why you feel the PR should not land, explain what those are. Do not
+expect to be able to block a Pull Request from advancing simply because you say
+"No" without giving an explanation. Be open to having your mind changed. Be open
+to working with the contributor to make the Pull Request better.
+
+Reviews that are dismissive or disrespectful of the contributor or any other
+reviewers are strictly counter to the Code of Conduct.
+
+When reviewing a Pull Request, the primary goals are for the codebase to improve
+and for the person submitting the request to succeed. **Even if a Pull Request
+does not land, the submitters should come away from the experience feeling like
+their effort was not wasted or unappreciated**. Every Pull Request from a new
+contributor is an opportunity to grow the community.
+
+### Review a bit at a time.
+
+Do not overwhelm new contributors.
+
+It is tempting to micro-optimize and make everything about relative performance,
+perfect grammar, or exact style matches. Do not succumb to that temptation.
+
+Focus first on the most significant aspects of the change:
+
+1. Does this change make sense for Tokio?
+2. Does this change make Tokio better, even if only incrementally?
+3. Are there clear bugs or larger scale issues that need attending to?
+4. Is the commit message readable and correct? If it contains a breaking change
+   is it clear enough?
+
+Note that only **incremental** improvement is needed to land a PR. This means
+that the PR does not need to be perfect, only better than the status quo. Follow
+up PRs may be opened to continue iterating.
+
+When changes are necessary, *request* them, do not *demand* them, and **do not
+assume that the submitter already knows how to add a test or run a benchmark**.
+
+Specific performance optimization techniques, coding styles and conventions
+change over time. The first impression you give to a new contributor never does.
+
+Nits (requests for small changes that are not essential) are fine, but try to
+avoid stalling the Pull Request. Most nits can typically be fixed by the Tokio
+Collaborator landing the Pull Request but they can also be an opportunity for
+the contributor to learn a bit more about the project.
+
+It is always good to clearly indicate nits when you comment: e.g.
+`Nit: change foo() to bar(). But this is not blocking.`
+
+If your comments were addressed but were not folded automatically after new
+commits or if they proved to be mistaken, please, [hide them][hiding-a-comment]
+with the appropriate reason to keep the conversation flow concise and relevant.
+
+### Be aware of the person behind the code
+
+Be aware that *how* you communicate requests and reviews in your feedback can
+have a significant impact on the success of the Pull Request. Yes, we may land
+a particular change that makes Tokio better, but the individual might just not
+want to have anything to do with Tokio ever again. The goal is not just having
+good code.
+
+### Abandoned or Stalled Pull Requests
+
+If a Pull Request appears to be abandoned or stalled, it is polite to first
+check with the contributor to see if they intend to continue the work before
+checking if they would mind if you took it over (especially if it just has nits
+left). When doing so, it is courteous to give the original contributor credit
+for the work they started (either by preserving their name and email address in
+the commit log, or by using an `Author: ` meta-data tag in the commit.
+
+_Adapted from the [Node.js contributing guide][node]_
+
+[node]: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md.
+[hiding-a-comment]: https://help.github.com/articles/managing-disruptive-comments/#hiding-a-comment