default all feature flags to off (#1811)

Changes the set of `default` feature flags to `[]`. By default, only
core traits are included without specifying feature flags. This makes it
easier for users to pick the components they need.

For convenience, a `full` feature flag is included that includes all
components.

Tests are configured to require the `full` feature. Testing individual
feature flags will need to be moved to a separate crate.

Closes #1791

1 files changed, 9 insertions, 0 deletions

diff --git a/tokio/README.md b/tokio/README.md
index 087ab025..530f0bac 100644
--- a/tokio/README.md
+++ b/tokio/README.md
@@ -52,6 +52,15 @@ an asynchronous application.
 
 ## Example
 
+To get started, add the following to `Cargo.toml`.
+
+```toml
+tokio = { version = "0.2.0", features = ["full"] }
+```
+
+Tokio requires components to be explicitly enabled using feature flags. As a
+shorthand, the `full` feature enables all components.
+
 A basic TCP echo server with Tokio:
 
 ```rust