write contribution guides

[rmnldwg]

Every repository in this organisation should have clear and easy-to-follow guidelines on how to collaborate on its content. This should cover both the members of the lycosystem organisation, as well as external contributors.

[rmnldwg]

It may make sense to write general rules - e.g. about code and commit style - into this organisation's wiki. The advantage of that would be that it does not have to be copy-pasted into every CONTRIBUTING.md of the individual repositories, I could just link to the wiki from these contribution guides.