Add CITATION.cff file

[schackartk]

Is your feature request related to a problem? Please describe.

I am unsure how to cite Black in my research articles.

It would be useful to have an explicit metadata component describing how to cite Black.

Describe the solution you'd like

A CITATION.cff file could be added to the root of the repository. This would allow GitHub to automatically generate a citation in both APA and BibTeX format. Additionally, reference manager plugins such as Zotero will be able to automatically capture the citation information.

Describe alternatives you've considered

The citation information could be added directly to the documentation, or an alternative citation format could be used such as BibTeX. But .cff seems to be the method recommended by GitHub.

Additional context

Similar citation files are used by popular libraries such as pandas and tensorflow, although some use BibTeX format instead such as NumPy. However, using BibTeX means that GitHub cannot automatically generate the citations (in APA) but instead just point to the citation file.