Add CITATION.cff and link with Zenodo

[rettinghaus]

It would be good to link Zenodo with this repository, so that every release automatically get's stored on Zenodo.
The metadata for that release can be stored in a CITATION.cff file.

Refers to #2670.

[peterstadler]

Yes and no.
A citation.cff file is probably good but an automated integration with Zenodo maybe not.
There's already a Zenodo archive at https://doi.org/10.5281/zenodo.3413524 with the release artifacts which is maintained manually. An automated integration would only archive the repository state without the release artifacts (to my knowledge).