Add CITATION.cff file

[n0toose]

Hello! I'm working on a thesis and briefly considered the use of the dashmap crate instead of HashMap. It would be helpful if it were possible to cite this crate in a way that is standardized (and, in the "open-source spirit") would also help the next person do the same without duplicating work.

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