enable easier citation by adding file to relevant HAPI repos

[jvandegriff]

Github has support for citation files that allows software to be cited easily.

Github support:
https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files

CFF file format:
https://citation-file-format.github.io

See this repo for an example:
https://github.com/Parsl/parsl
(See the "Cite this repository" link on the side)