docs(contributing): Mention git commit guidelines
docs(contributing): Mention git commit guidelines

shane-tomlinson/fxa-auth-server,dannycoates/picl-idp,mozilla/fxa-auth-server,mozilla/fxa-auth-server,mozilla/fxa-auth-server,dannycoates/picl-idp,dannycoates/picl-idp,TDA/fxa-auth-server,eoger/fxa-auth-server,eoger/fxa-auth-server,eoger/fxa-auth-server,TDA/fxa-auth-server,TDA/fxa-auth-server,shane-tomlinson/fxa-auth-server,shane-tomlinson/fxa-auth-server,shane-tomlinson/fxa-auth-server,eoger/fxa-auth-server,dannycoates/picl-idp,TDA/fxa-auth-server,mozilla/fxa-auth-server