Handle CONTRIBUTING.md

[annevk]

A bunch of standards have https://github.com/whatwg/url/blob/main/CONTRIBUTING.md which seems like something we should handle here instead.

[domenic]

We could maybe move it into .github, per https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors . Since they're just redirect files anyway, I wonder if cordoning them off is nicer than having them top-level.