RFC: Contribution guidelines

[coolsoftwaretyler]

We haven't updated our contribution guidelines in four years. It's time to dust them off a little bit.

We'll definitely be moving these into the GitHub convention contributing.md document so they show up in the repo, but other than that change, I'd love to hear some suggestions from the community about what we should and should not include in our contribution guidelines. For example:

1. Should we make tests a requirement of all PRs? Should there be exceptions for things like docs? Should we not require this at all
2. How can new contributors find good issues? Should we document our tags, or have more general advice?
3. How will we resolve different opinions on implementations and what goes in or out of the library?
4. What kind of PR descriptions will we require, if any?
5. Are there any great contributing guidelines from other open source projects you love? If so, what are they? Why do you love them?
6. Any contributing guidelines you dislike? If so, what do you dislike about them?

I'll keep this open for a few weeks and keep folks updated as we make progress here.

[coolsoftwaretyler]

Shipped! #2090