Added note to readme

README.md

@@ -425,6 +425,26 @@ Example of payload that is sent to FCM server:
 
 Check local notification guide below for configuration.
 
+**IMPORTANT**: When using the `admin.messaging` API, you need to `JSON.stringify` the `custom_notification` value:
+
+```
+let tokens = [...];
+let payload = {
+  data: {
+    custom_notification: JSON.stringify({
+      body: 'Message body',
+      title: 'Message title'
+      ...
+    })
+  }
+};
+let options = { priority: "high" };
+
+admin
+  .messaging()
+  .sendToDevice(tokens, payload, options);
+```
+
 ### Behaviour when sending `notification` and `data` payload through GCM
 - When user clicks notification to **launch** the application, you can get that notification by calling `FCM.getInitialNotification`. (NOTE: reloading javascript or resuming from background won't change the value)