add release documentation to CONTRIBUTING.md (#1171)

## Motivation

Currently, the process for releasing a new version of a Tokio crate is
somewhat complex, and is not well-documented. To make it easier for
contributors to release minor versions more frequently, there should be
documentation describing this process.

## Solution

This branch adds a section to `CONTRIBUTING.md` describing how to
release a new version of a Tokio crate. The steps are based on those
described by @carllerche in an offline conversation.

I've also added a quick shell script to actually publish new crate 
versions. This should make it harder to make mistakes when 
publishing.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

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