Recommend CITATION.cff files rather than adding tex-style plaintext

The current best-practice for citations seems to be to go with the format described [in the github docs](https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files) and [CFF format](https://citation-file-format.github.io/) rather than with a tex-style citation file.
At least we could mention that this is a thing.