Tell docs.rs about the yaml feature

All the other features are either irrelevant, for nightly or already included by default so this should build with all features.

https://github.com/onur/docs.rs/pull/73

Fixes clap-rs#73

Author: Marwes

Cargo.toml

@@ -82,3 +82,6 @@ rpath = false
 lto = false
 debug-assertions = true
 codegen-units = 4
+
+[package.metadata.docs.rs]
+features = ["yaml"]