Create citation file for Zenodo DOI

[mkavulich]

Description

CCPP is seeking to provide DOIs for all of its various repositories so that they can be cited scientifically. Github conveniently provides a standard way to include this information, via a top-level "CITATION.cff" file. We plan to create a DOI for the CCPP framework code via the Zenodo website for the next release, and include that information in this file.

As an example, see the citation file in the SCM repository: https://github.com/NCAR/ccpp-scm/blob/main/CITATION.cff

Solution

Add a CITATION.cff file at the top-level once the DOI is available, prior to the next CCPP SCM release.