Document the authentication formats

Original idea from @librelois: https://forum.duniter.org/t/duniter-scuttlebot-comment-partager-les-clefs-de-chiffrage-des-crypto-applications/6775/6?u=moul

Document somewhere the authentication formats generated and used by Silkaj.

This could be a common documentation shared by the Python software.

This document could be stored into DuniterPy, and Silkaj documentation could have a link to it and also copy it into its documentation (website).

This also differ of current authentication generator and reading (0.7.3) and the futures coming with #173 (closed).