add contributing.md
add contributing.md

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