Add a bibtex entry to the readme

[maringuu]

It would be nice to have a standard way to cite the usage of FACT in publications.

[jstucke]

There is actually a standardized way of doing this: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files

I'm not really sure what we would fill it with, though, Feel free to open a PR