Add example for multicast on routeNotificationForFcm() (#71)

README.md

@@ -98,7 +98,7 @@ class User extends Authenticatable
     /**
      * Specifies the user's FCM token
      *
-     * @return string
+     * @return string|array
      */
     public function routeNotificationForFcm()
     {
@@ -107,6 +107,27 @@ class User extends Authenticatable
 }
 ```
 
+You can also return an array of tokens to send notifications via multicast to different user devices.
+
+```php
+class User extends Authenticatable
+{
+    use Notifiable;
+
+    ....
+
+    /**
+     * Specifies the user's FCM tokens
+     *
+     * @return string|array
+     */
+    public function routeNotificationForFcm()
+    {
+        return $this->getDeviceTokens();
+    }
+}
+```
+
 Once you have that in place, you can simply send a notification to the user by doing the following:
 
 ```php