adding Citation files (CFF) to cookiecutter-data-science template

[kjgarza]

With the upcoming release of v2, I think this would be a nice addition.

With the addition of this CFF file Github enables academics and researchers to let people know how to correctly cite their work, especially in academic publications/materials. Originally proposed by the research software engineering community, CITATION.cff files are plain text files with human- and machine-readable citation information. When we detect a CITATION.cff file in a repository, we use this information to create convenient APA or BibTeX style citation links that can be referenced by others.

This can be done easily with just a lines, as seen in my branch here (and its corresponding #274)