Adding note about security/validity of ID token parser.
Adding note about security/validity of ID token parser.

GoogleCloudPlatform/appengine-endpoints-tictactoe-python,chenbi/GoogleAppEngine,opozo/web-endpoints,pnedunuri/appengine-endpoints-tictactoe-python,pnedunuri/appengine-endpoints-tictactoe-python,chenbi/GoogleAppEngine,opozo/web-endpoints,pnedunuri/appengine-endpoints-tictactoe-python,opozo/web-endpoints,opozo/web-endpoints,opozo/web-endpoints,GoogleCloudPlatform/appengine-endpoints-tictactoe-python,opozo/web-endpoints