add pull request "requirements" to documentation

I don't really like the word "requirements" (is there a better word?), but there are some things that pull requests should have and we should write those down so we can point people at them.
- tests/lint should pass (`npm test`)
- Javascript changes should have tests
- documentation?
- other things?

It might also be good to a have a pointer to [codeforboston/lgbtq-data](https://github.com/codeforboston/lgbtq-data) for data changes.