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

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