docs tweaks

[maelle]

• Create small README.md
• Move REQUIREMENTS.md to .github/CONTRIBUTING.md

happy to make a PR

[szhorvat]

Can we keep the contribution docs at the top level please? If it's in a subdirectory, I will surely forget where to look for it ...

[maelle]

yes CONTRIBUTING.md at the root would work, it'd still be a standard location https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors 😸