Add CONTRIBUTING.md to this repository

[svrnm]

While we establish the ways we work and operate, we should aim to write these things down (e.g. in this issue) and eventually have them reflected in the README.md or a dedicated CONTRIBUTING.md

[svrnm]

I introduced labels for:* to tag issues in a way that they indicate for whom in the contributor journey this is, so for:maintainer means this is focused around a problem that maintainers may have, or for:newcomers is an issue that is focused around a problem that newcomers have