rework contributing.md
rework contributing.md

enolfc/oauthenticator,willjharmer/oauthenticator,zonca/oauthenticator,jupyter/oauthenticator,Xarthisius/oauthenticator,santi81/oauthenticator,yuvipanda/mwoauthenticator,zonca/cilogonauthenticator,cmclaughlin/oauthenticator,yuvipanda/mwoauthenticator,luisfdez/oauthenticator,NickolausDS/oauthenticator,minrk/oauthenticator,jupyterhub/oauthenticator,jupyter/oauthenticator,santi81/oauthenticator,Xarthisius/oauthenticator,zonca/cilogonauthenticator,zonca/oauthenticator,cmclaughlin/oauthenticator,luisfdez/oauthenticator,maltevogl/oauthenticator,willjharmer/oauthenticator