Update setting-up-google-firebase-cloud-messaging-server.md

Added instructions for developerapp

Author: Jonathan Diehl

content/en/docs/refguide/mobile/using-mobile-capabilities/push-notifications/setting-up-google-firebase-cloud-messaging-server.md

@@ -66,6 +66,10 @@ For a native mobile app, your bundle ID will be what you specify for the **App i
 * For Android, open *android/app/build.gradle* and consult **applicationId** to find your ID 
 * For iOS, open *ios/Config/config.xcconfig* and consult **BUNDLE_IDENTIFIER** to find your ID
 
+{{% alert color="info" %}}
+If you also want to support a custom developer app, repeat above steps and append `.developerapp` to the IDs.
+{{% /alert %}}
+
 Click **Deploy**, then **Mobile App**. Your ID is listed as **App Identifier**:
 
 {{< figure src="/attachments/howto/mobile/native-mobile/implementation/notifications/setting-up-google-firebase-cloud-messaging-server/hybrid_app_identifier.png"   width="350"  >}}