Add `CITATION.cff` metadata file for referencing CFA standalone

[sadielbartholomew]

Add CITATION.cff metadata file, which is a relatively-new standard for providing means to cite a repository.

Though the intention is to get CFA incorporated into the CF Conventions, with community approval and a PR in preparation at this stage, we agreed it would be useful to also enable citation of CFA on its own (e.g. because there won't be a specific chapter/section in the CF Conventions document dedicated to CFA, it will be referenced instead here and there as relevant), and this file will allow one to do so.

@davidhassell and any other reviewers, please note:

• I used this dedicated site to create and validate the file, skipping some optional fields which didn't seem that useful or relevant in this case, but feel free to try it out in case you want o add or remove info I have suggested here in this PR version of the file. One particular field that might be useful is to specific 'version specific' details to tie to a given version/commit, but it looks like there have been plenty of commits since our last release so even if we did want that, I wouldn't be sure what to specify.
• I have tried to ensure correct author info (ORCID and affiliations) from what I can find online, but these should probably be double-checked by everyone involved.

and feel free to suggest any changes e.g. different keywords and abstract.

[sadielbartholomew]

Oh and I checked about how to best deal with authors with multiple affiliations, but it looks like that hasn't been decided yet and the value can just be a string, hence I just used 'and' (old school!).

[sadielbartholomew]

Thanks for reviewing, David. Merging...