Add comment telling users to use latest

Author: dlarocque

messaging/service-worker.js

@@ -10,6 +10,7 @@ function initInSw() {
   // Give the service worker access to Firebase Messaging.
   // Note that you can only use Firebase Messaging here. Other Firebase libraries
   // are not available in the service worker.
+  // Replace 10.13.2 with latest version of the Firebase JS SDK.
   importScripts('https://www.gstatic.com/firebasejs/10.13.2/firebase-app-compat.js');
   importScripts('https://www.gstatic.com/firebasejs/10.13.2/firebase-messaging-compat.js');