Link to rustc guide

As proposed in rust-lang/rustc-dev-guide#239

Author: mark-i-m

src/doc/index.md

@@ -104,3 +104,9 @@ Rust. It's also sometimes called "the 'nomicon."
 ## The Unstable Book
 
 [The Unstable Book](unstable-book/index.html) has documentation for unstable features.
+
+## The `rustc` Contribution Guide
+
+[The `rustc` Guide](https://rust-lang-nursery.github.io/rustc-guide/) documents how
+the compiler works and how to contribute to it. This is useful if you want to build
+or modify the Rust compiler from source (e.g. to target something non-standard).