update yaml example comments
// Using yaml requires calling a clap macro `load_yaml!()` so we must use the '#[macro_use]'
// directive
+// Note: If you're using clap as a dependency and don't have a feature for your users called
+// "yaml", you'll need to remove the #[cfg(feature = "yaml")] conditional compilation attribute
#[cfg(feature = "yaml")]
#[macro_use]
extern crate clap;
