CONTRIBUTORS document

[bighappyface]

It's probably time we look at adding a CONTRIBUTORS document to the repo root to help set the tone for all that wish to participate.

Some ideas on the contents:

1. Criteria for an acceptable PR
2. Checklist/guidance for issues
3. Checklist/guidance for code reviews (could be another document)
4. List of contributors at the bottom (to replace the authors in composer.json)

[mathroc]

given the first 3 points, it should probably be named CONTRIBUTING.md

[DannyvdSluijs]

Also a PR template could help out in this context, see https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository in which we can also refer to this type of document.

Also something like https://allcontributors.org/ could help out in recognizing all contributors.