Add CONTRIBUTING.md

[ursjoss]

See

• https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors
• https://github.com/github/docs/contribute
• https://github.com/github/docs/blob/774b006d1bfe270fa98b6e0924427a094b5fa3eb/CONTRIBUTING.md