[Doc] Add documentation for contributing users
Changes follow the draft in https://docs.google.com/document/d/1ka1rBASGXDJ1ibU7O-5S6ue_na0EXZicYi4GZCtYnA0/edit?usp=sharing
Prochaines maintenances programmées: mardi 02/04, lundi 06/05, lundi 03/06
Pour plus d'informations: https://doc-si.inria.fr/display/SU/Gitlab
Changes follow the draft in https://docs.google.com/document/d/1ka1rBASGXDJ1ibU7O-5S6ue_na0EXZicYi4GZCtYnA0/edit?usp=sharing