Add citation file

[fernandohds564]

This repo also has the cppcolleff package. Maybe should these files be placed inside pycolleff?

[murilobalves]

This repo also has the cppcolleff package. Maybe should these files be placed inside pycolleff?

Initially that was the idea, but citation file must be in the root folder of the repository for Github citation tool to work

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

Maybe include cppcolleff in the description of the citation file?

[fernandohds564]

Yes, I think it is a good idea to also include cppcolleff in the description.