Feature request: make it easier to cite DART software

[nancycollins]

github feature

i just heard a talk where they mentioned that if you add a 'citation.cff' file to the main branch of your github repo, github will automatically create a 'cite this software' widgit. users click on that and it shows them how to cite your software. i guess you put the info in that file for what the citation info is. seems helpful/useful.

[hkershaw-brown]

@ann-norcio

Here is the citing info from the README.md

Citing DART
To cite DART, please use the following text:

The Data Assimilation Research Testbed (Version X.Y.Z) [Software]. (2021). Boulder, Colorado: UCAR/NCAR/CISL/DAReS. http://doi.org/10.5065/D6WQ0202

and update the DART version and year as appropriate.

[hkershaw-brown]

added in #658