Add RP paper citation to the radical.pilot repository

[AymenFJA]

I would like to add the following:

Add radical.pilot citation file to the repository. Easy citation and easy access for RP papers.

[eirrgang]

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