Merge #202

202: Document feature flags in readme r=yoshuawuyts a=yoshuawuyts

Follow up to #190, this documents our feature flags in our readme. Thanks!

Co-authored-by: Yoshua Wuyts <yoshuawuyts@gmail.com>
Co-authored-by: Yoshua Wuyts <yoshuawuyts+github@gmail.com>

Author: bors[bot]

README.md

@@ -81,6 +81,26 @@ fn main() {
 }
 ```
 
+## Features
+
+`async-std` is strongly commited to following semver. This means your code won't
+break unless _you_ decide to upgrade.
+
+However every now and then we come up with something that we think will work
+_great_ for `async-std`, and we want to provide a sneak-peek so you can try it
+out. This is what we call _"unstable"_ features. You can try out the unstable
+features by enabling the `unstable` feature in your `Cargo.toml` file:
+
+```toml
+[dependencies]
+[dependencies.async-std]
+version = "0.99"
+features = ["unstable"]
+```
+
+Just be careful when using these features, as they may change between
+versions.
+
 ## Take a look around
 
 Clone the repo: