Make repository directly citeable

Created by: asogaard

So far, we have configured the repo to crate DOIs on Zenodo on each new tag (https://zenodo.org/record/6720188), and Zenodo exposes a citation that can be used by people using the repo. However, it is also possible to add a CITATION file (https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files) to provide a convenient citation right from the main page of the repo.