Create a CONTRIBUTING.md file #434

There's a common pattern on GitHub and in open source software more generally to create a CONTRIBUTING.md file which explains how to contribute to a repository. We ought to have that for this repository, including steps like those identified in #429 that go beyond just editing the schema. I am happy to draft one, but it'll need review from folks who have been involved longer than I have to make sure that I haven't misstated expectations or missed steps.