Add CITATION.cff - Githubissues

[x] I reviewed the Contributing Guide.
[x] I reviewed the cFS README.md file to see if the feature is in the major future work.
[x] I performed a cursory search to see if the feature request is relevant, not redundant, nor in conflict with other tickets.

Is your feature request related to a problem? Please describe. It would be helpful to be able to quickly get the citation for cFS and related.

Describe the solution you'd like Add a CITATION.cff so that citations can be quickly copied.

Describe alternatives you've considered None

Additional context Citation file format reference: https://citation-file-format.github.io/

Requester Info Andrew Liounis: NASA/GSFC

nasa / cFS