GH: Add CITATION.cff
Enables a citation widget on the side of the repository that provides a copy-pastable ABA & Bibtex citation: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files

Followed this schema: https://github.com/citation-file-format/citation-file-format/blob/1.2.0/schema-guide.md

check that this displays ok :-> <img width="352" alt="Screen Shot 2022-07-16 at 10 35 16 AM" src="https://user-images.githubusercontent.com/10647082/179365944-b9dc88ce-85fa-4b3f-a2d6-bb7728e34305.png">
