Add CITATION.cff

[tmigot]

I am reviewing the CITATION usage in the repos and in particular moving to CFF files. The bib can still be generated using Github menu.
Here are a couple of suggestions to adapt this repo:

• use CITATION.cff instead of CITATION.bib ; a good example is https://github.com/JuliaSmoothOptimizers/CaNNOLeS.jl/blob/main/CITATION.cff and/or you can use this tool https://citation-file-format.github.io/cff-initializer-javascript/#/
• update the link in the README.md after that