add CITATION.cff file to repo?

[mnlevy1981]

This was mentioned over in mom-ocean/MOM6: there's a standard format for including a citation in a github repository. I was thinking it might be a good way to direct people to Long et al. (2021). Another option would be to register each release tag for a DOI, but I think there would be some circular dependencies preventing us from including the correct DOI in the tag itself (we may need a process like create marbl1.0.0, give it a DOI, and then immediately create marbl1.0.1 with a CITATION.cff pointing to the 1.0.0 DOI).