[google_sign_in] Document App Check setup in iOS README #168827
Description
Following up from #166967, the plugin is now capable of supporting App Check, but it requires native setup, which we should document in the README.
(Wrapping the API with a Dart call is an option, but the instructions specifically say to do it as early as possible, and we have precedent—such as googe_maps_flutter API key setup—for documenting things that SDKs recommend putting in application:didFinishLaunching: rather than wrapping them and having them be called later.)