doc: Enable lib doc in more cases

Before, we limited it to the `doc` feature because of how extensive the
README was.  It has since scaled back, so we can be more selective with
what features enable it.

src/lib.rs

@@ -4,7 +4,7 @@
 // notice may not be copied, modified, or distributed except according to those terms.
 
 #![doc(html_logo_url = "https://raw.githubusercontent.com/clap-rs/clap/master/assets/clap.png")]
-#![cfg_attr(feature = "unstable-doc", doc = include_str!("../README.md"))]
+#![cfg_attr(feature = "derive", doc = include_str!("../README.md"))]
 //! <https://github.com/clap-rs/clap>
 #![warn(
     missing_docs,