Add citation information in gh-supported format

[mihai-sysbio]

Description of the issue:

GitHub has added support for citations. To make this work in the repo, a citation.cff file should be added. For more info on the file format, see citation-file-format.

Expected feature/value/output:

A citation.cff file in the repository.

Current feature/value/output:

Citation information shown in the Readme.

I hereby confirm that I have:

• Checked that a similar issue does not exist already

[haowang-bioinfo]

@mihai-sysbio sounds good, just go ahead

[edkerk]

Included since 2.0.0