[request] Add `CONTRIBUTING.md`

It would be great if there were a guide for contributors.

Questions I'm running into as I'm preparing a PR, which could be covered in such a guide:

• What's your convention around PR titles?
• Is there anything in particular you want in the PR description (more specific than the general "why and how")?
• Are there any conventions we should stick to in the code?
• What kinds of tests should be added?
  • What's the overall idea of the various kinds of tests which are already there?
  • How are the webpack wrappers in the unit tests generated, and when is that the appropriate kind of test to write?
  • When do special cases need to be added to unit.test.js?
  • etc

Related to that last set of questions, it would also be super helpful if comments could be added to the test files (integration.test.js, unit.test.js, etc). As it stands, it's pretty hard to figure out what's going on and why, and therefore how new tests should be written.

Thanks!

(For context, I am currently working on https://github.com/vercel/nft/pull/322.)

Start with the requested CONTRIBUTING.md and review integration.test.js, unit.test.js, and the existing webpack wrappers to understand the test categories and conventions. Done means the guide answers the listed PR, code, and testing questions and the relevant test files explain how and why new tests should be written.