Auto merge of #988 - fitzgen:overview, r=emilio
Auto merge of #988 - fitzgen:overview, r=emilio

Add an architectural overview of `bindgen` to CONTRIBUTING.md

This should help new contributors who are coming to the code base for the first time get up and running.

r? @emilio

emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen