ENH: Add a CITATION file

[ljlamarche]

Requested feature

Adding a CITATION file to the repo root seem to be a convenient way to tell users how they should cite the software.

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

The problem or gap this feature will address

In the interest of good open-software stewardship, we should encourage people to properly cite our software when they use it.

Possible alternatives

Add information about how to cite in the standard documentation. (I don't see why we couldn't do both!)