Add CITATION.cff

[dweindl]

Closes #572

[dilpath]

Looks fine to me.

• could add a URL to the publication, but the DOI suffices
• could add a license, though I guess it's irrelevant for a citation
• I can't see a link to the paper in the README. Could move the "How to cite" section of the docs to the README, or mention the paper in the README somewhere
• Is the "How to cite" section redundant or complementary to this?

[dweindl]

* could add a URL to the publication, but the DOI suffices

GitHub automatically converts the doi to an URL, so I think that's fine.

* could add a license, though I guess it's irrelevant for a citation

will do.

* I can't see a link to the paper in the README. Could move the "How to cite" section of the docs to the README, or mention the paper in the README somewhere

* Is the "How to cite" section redundant or complementary to this?

I think it's good to have it somewhere on the RTD page, but the README itself doesn't really need it.