Add instruction for downloading service account json file.

Author: maratal

src/pages/docs/push/getting-started/react-native.mdx

@@ -49,7 +49,7 @@ Firebase Cloud Messaging delivers push notifications on both Android and iOS. To
 4. For iOS, upload your APNs authentication key to Firebase: In the Firebase Console, go to **Project Settings** → **Cloud Messaging** → **APNs Authentication Key** → **Upload**.
 5. In the Ably dashboard, navigate to your app's **Notifications** tab.
 6. Scroll to the **Push Notifications Setup** section and select **Configure Push**.
-7. Follow the instructions to upload your Firebase service account JSON file.
+7. Follow the instructions to upload your Firebase service account JSON file (you can download it from your Firebase console: **Project configuration** → **Service Accounts** → **Generate new private key**).
 
 ### Create a React Native project <a id="create-project"/>