fill out contributing

Author: steveklabnik

src/doc/rustc/src/contributing.md

@@ -1 +1,6 @@
 # Contributing to rustc
+
+We'd love to have your help improving `rustc`! To that end, we've written [a
+whole book](https://rust-lang-nursery.github.io/rustc-guide/) on its
+internals, how it works, and how to get started working on it. To learn
+more, you'll want to check that out.