Add contributor guidelines

[ehuss]

It is usually best practice to include a CONTRIBUTING.md file with guidelines for contribution. What should those guidelines be? Do we want to steer people wanting to write new content to first work with the team? Probably should point them at the authoring guide, but what else do they need to know?

[ehuss]

Closing as this was related to having a separate spec, and contributing guidelines are available at https://github.com/rust-lang/reference/blob/master/CONTRIBUTING.md