Add page or section on getting PRs accepted

- Keep PRs small
- Run linting
- Run formatting
- Run the tests

Useful guide: https://docs.djangopackages.org/en/latest/contributing/#how-to-get-your-pull-request-accepted

Start by locating the repository's contributor documentation and review the linked guide on getting pull requests accepted. Add a page or section covering small PRs, linting, formatting, and tests; done means all four recommendations are clearly documented.

Written by the indexing model from the issue text.