Add Code of Conduct

[Conengmo]

See https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/adding-a-code-of-conduct-to-your-project on how to do this.

We can use the default template, since I suspect we don't have any special needs on this topic. We can also lift some ideas from Geopandas for example.

[ocefpaf]

I like this one and use it in other projects: https://www.contributor-covenant.org/