Add gatewayapi-notifier

Author: PGLongo

notifier.rst

@@ -60,6 +60,7 @@ Service     Package                           DSN
 Esendex     ``symfony/esendex-notifier``      ``esendex://USER_NAME:PASSWORD@default?accountreference=ACCOUNT_REFERENCE&from=FROM``
 FreeMobile  ``symfony/free-mobile-notifier``  ``freemobile://LOGIN:PASSWORD@default?phone=PHONE``
 Infobip     ``symfony/infobip-notifier``      ``infobip://TOKEN@default?from=FROM``
+GatewayAPI  ``symfony/gatewayapi-notifier``   ``gatewayapi://TOKEN@default?from=FROM``
 Mobyt       ``symfony/mobyt-notifier``        ``mobyt://USER_KEY:ACCESS_TOKEN@default?from=FROM``
 Nexmo       ``symfony/nexmo-notifier``        ``nexmo://KEY:SECRET@default?from=FROM``
 OvhCloud    ``symfony/ovhcloud-notifier``     ``ovhcloud://APPLICATION_KEY:APPLICATION_SECRET@default?consumer_key=CONSUMER_KEY&service_name=SERVICE_NAME``