Add citation files to relevant repositories

[snomos]

As suggested by @unhammer. See the reference documentation, and an example.

@lynnda-hill @flammie @Trondtr and others.

[flammie]

aye, I usually do this lazily when at conferences e.g. https://github.com/apertium/apertium-sme-deu/blob/master/CITATION.cff but from discussion in IRC I understand that maybe this can simply be used with "releases" and then optionally with zenodo references; that is, its intent is to provide software and data citations first and the academic paper citation is just one special case.

[unhammer]

added:

• https://github.com/apertium/apertium-sme-nob/blob/master/CITATION.cff
• https://github.com/divvun/libdivvun/blob/master/CITATION.cff

(though feel free to edit if there are other more relevant things that should be there or additional info to append)