Add Paper Citation

It would be useful to add a link to the paper (DOI: https://doi.org/10.1063/5.0147903) for PluginPlay in the repo, with an accompanying "How to Cite" section at the end. If you get the chance, a [`CITATION.cff`](https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files) file would make it even easier to cite.