Add CONTRIBUTING.md

[rosvik]

There should be a CONTRIBUTING.md file to encourage external PRs 🤗

Some resources for creating one:

• https://github.com/nayafia/contributing-template
• https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors
• https://mozillascience.github.io/working-open-workshop/contributing/