Meta: ease contributing to bindgen

[fitzgen]

Ramping up on bindgen as a newcomer is harder than it should be (eg #51 (comment)) so we should make it easier. This will serve as a meta bug for such issues.

Will add more here as file more issues.

• Make cargo test work: Make 'cargo test' work #51
• Add a CONTRIBUTING.md file w/ an overview of bindgen's architecture: Add CONTRIBUTING.md file #93
• Add #![deny(missing_docs)]: Add #![deny(missing_docs)] #94
• Don't run tests expected to fail if --features llvm_stable is missing: Don't run tests that are expected to fail if --features llvm_stable is not supplied #97

[fitzgen]

I think we can close this now :)

Hopefully it is a lot easier to get started hacking on bindgen now :)