Adds CITATION.cff file to make this repository citeable

[basicavisual]

This PR adds citation capabilities to this repository through a CITATION.cff. This is a file type supported by github, zenodo and zotero.

After merge, bibtex renders as follows:

@software{Gonzalez-Beltran_jsonldschema,
author = {Gonzalez-Beltran, Alejandra and Sansone, Susanna-Assunta and Batista, Dominique},
license = {BSD-3-Clause},
title = {{jsonldschema}},
url = {https://github.com/FAIRsharing/jsonldschema}
}

More info:
https://citation-file-format.github.io/

https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files