Add link to /about/metadata

Author: jyn514

metadata/lib.rs

@@ -3,6 +3,8 @@
 //! Collect information that allows you to build a crate the same way that docs.rs would.
 //!
 //! This library is intended for use in docs.rs and crater, but might be helpful to others.
+//! See <https://docs.rs/about/metadata> for more information about the flags that can be set.
+//!
 //! Here is an example use of the crate:
 //!
 //! ```