Add CITATION.cff file

[robyngit]

GitHub supports citation metadata via a CITATION.cff file. To improve discoverability and compatibility with citation tools, we should add a CITATION.cff file that references the existing DOI.

GitHub uses this file to show a "Cite this repository" button like so:

This file could also be used to help automate the release workflow (#2593) - it would give us a clean way to get the current DOI to download the existing datacite xml to update. In turn, the automated workflow could also update the CITATION.cff on new releases.

[mbjones]

Thanks, this is a great idea. The CFF metadata spec is a subset of the metadata in the CodeMeta (https://codemeta.github.io) software metadata spec that we have helped develop, and which GitHub originally supported for citation metadata. If Gtihub still supports codemeta.json for citation metadata for software, I'd prefer to use that format as it is schema.org compatible and is the direction that other groups like scicodes.net have adopted. Can you take a look at that as well when implementing here please?