Document feature gate checking 

There is a [blank chapter](https://github.com/rust-lang/rustc-dev-guide/blob/master/src/feature-gate-ck.md) on feature gate checking that should either be filled out or removed. 

There is a small mention of how to check whether a [feature is enabled](https://github.com/rust-lang/rustc-dev-guide/blob/master/src/implementing_new_features.md#stability-in-code), but this might not be sufficient.