Update README.md (#141)

Author: Sml995

README.md

@@ -54,7 +54,8 @@ Add your OneSignal App ID and REST API Key to your `config/services.php`:
 ...
 'onesignal' => [
     'app_id' => env('ONESIGNAL_APP_ID'),
-    'rest_api_key' => env('ONESIGNAL_REST_API_KEY')
+    'rest_api_key' => env('ONESIGNAL_REST_API_KEY'),
+    'guzzle_client_timeout' => env('ONESIGNAL_GUZZLE_CLIENT_TIMEOUT', 0),
 ],
 ...
 ```