[request] Add `CONTRIBUTING.md`

[lobsterkatie]

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 #322.)

[styfle]

• PR titles don't have a convention and PR descriptions just need a why and how like you said.
• Not really any conventions, we should probably add style-guide as a dev dep to automate conventions.
• Depending on the change, unit tests are great and integration great, and both are even better.
• The webpack wrappers were just copied from the output of a webpack app.
• The special cases are just quick hacks, they really only need to be added if there isnt an easy solution for static analysis.
• The tests are usually just adding a test fixture as a new directory, and you rarely have to modify the .test.js files. The input.js and output.js are the only special files in each fixture.