feat(push): document the format of the push payloads
feat(push): document the format of the push payloads

mozilla/fxa-auth-server,eoger/fxa-auth-server,shane-tomlinson/fxa-auth-server,shane-tomlinson/fxa-auth-server,TDA/fxa-auth-server,TDA/fxa-auth-server,mozilla/fxa-auth-server,mozilla/fxa-auth-server,shane-tomlinson/fxa-auth-server,TDA/fxa-auth-server,mozilla/fxa-auth-server,shane-tomlinson/fxa-auth-server,eoger/fxa-auth-server,eoger/fxa-auth-server,eoger/fxa-auth-server,TDA/fxa-auth-server