add license for the repo

[StroemPhi]

This repo has no license at the moment.
MOP and RXNO are licensed under CC-BY 4.0.

I'm not sure what the proper OA license would be for the repo. RO uses CC-0 and OBI, IAO and CHEMINF have also no license for the repo. I guess the most important part is having the release artefacts licenced, but I think it would be in the best interest of open science to also declare an open licence for the repo. This would be easier to reuse documentation and other supplementary data in here in other research contexts, like workshops etc.