Added information about authentication providers in default_config.py
    
    Came across this at https://github.com/pypingou/pagure/pull/21.
    I added a reference in default_config.py to Documentation which gives
    information about various authentication providers.
    
    Signed-off by: Mohana Datta Yelugoti <ymdatta@protonmail.com></ymdatta@protonmail.com>
    
    Merges https://pagure.io/pagure/pull-request/3404
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>