Add CITATION.cff file

[mikemhenry]

See https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files for more info.

The format is really nice in that it provides a "recommended citation" so that we can point people to cite the repo via zenodo + this paper https://choderalab.squarespace.com/publications/2014/4/26/identifying-ligand-binding-sites-and-poses-using-gpu-accelerated-hamiltonian-replica-exchange-molecular-dynamics

See also https://github.com/choderalab/yank/issues/1210