Add a link to an RFC cross reference

To aid future readers

Author: sgrif

text/0769-sound-generic-drop.md

@@ -4,9 +4,11 @@
 
 # History
 
-2015.09.18 -- This RFC was partially superceded by RFC 1238, which
+2015.09.18 -- This RFC was partially superceded by [RFC 1238], which
 removed the parametricity-based reasoning in favor of an attribute.
 
+[RFC 1238]: https://github.com/rust-lang/rfcs/blob/master/text/1238-nonparametric-dropck.md
+
 # Summary
 
 Remove `#[unsafe_destructor]` from the Rust language.  Make it safe