docs(messaging): Note about keeping sw up to date (#2553)

Add warning missmatch firebase version could lead unable to get notification

Author: hiepxanh

docs/messaging/messaging.md

@@ -50,6 +50,34 @@ You can either use the `firebase-messaging-sw.js` file provided in the docs or y
   ],
 ```
 
+[Warning] Remember update the `firebase-messaging-sw.js` everytime you update the `firebase` in package.json. The missmatch version could lead to unable to receive notification in `foreground`, you can create your `firebase-messaging-sw.js` like this:
+
+```js
+// 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.
+importScripts('https://www.gstatic.com/firebasejs/[the number of version matching with firebase in package.json]/firebase-app.js');
+importScripts('https://www.gstatic.com/firebasejs/[for example: 7.16.1]/firebase-messaging.js');
+
+// Initialize the Firebase app in the service worker by passing in the
+// messagingSenderId.
+
+firebase.initializeApp({
+    apiKey: '<your-key>',
+    authDomain: '<your-project-authdomain>',
+    databaseURL: '<your-database-URL>',
+    projectId: '<your-project-id>',
+    storageBucket: '<your-storage-bucket>',
+    messagingSenderId: '<your-messaging-sender-id>'
+});
+
+// Retrieve an instance of Firebase Messaging so that it can handle background
+// messages.
+const messaging = firebase.messaging();
+
+
+```
+
 ### Requesting permission
 
 Once you have the Firebase Messaging Service Worker set up and installed, you need to request permission to send a user notifications. While the browser will popup a UI for you, it is highly recommend to ask the user for permission with a custom UI and only ask when it makes sense. If you blindly ask for permission, you have an extremely high chance of getting denied or blocked.