Add CITATON.cff

[matthewfeickert]

As @ekourlit pointed out,

cabinetry/README.md

Line 75 in 8f78894

`cabinetry` is also described in a paper submitted to vCHEP 2021: [10.5281/zenodo.4627037](https://doi.org/10.5281/zenodo.4627037).

, is old as https://doi.org/10.1051/epjconf/202125103067 is the DOI that should be used now, but this should all be controlled programmatically through a CITATION.cff file.

[alexander-held]

Good catch! This would also be a good opportunity to make the phrasing in the readme a bit more clear along the lines of "if you find cabinetry useful in your research, please consider citing it: [reference]".