Add citation.cff

To increase, improve findability and make citation easier we should also implement this format

https://citation-file-format.github.io/
https://github.com/citation-file-format/citation-file-format#tools-to-work-with-citationcff-files-wrench

There's even a converter which can export the data to other formats like CodeMeta. https://github.com/citation-file-format/cff-converter-python

Although there are some projects to automatically extract data from pypi and other sources of a GitLab / GitHub repo, for now the best solution is probably still to maintain this by hand (and maybe update the Codemeta file