Update top-level-api.md

docs/top-level-api.md

 ```js
 Navigation.registerScreen('example.AdvancedScreen', () => AdvancedScreen);
 ```
+
+## getCurrentlyVisibleScreenId()
+In some cases you might need the id of the currently visible screen. This method returns the unique id of the currently visible screen:
+`const visibleScreenInstanceId = await Navigation.getCurrentlyVisibleScreenId()`
+In order to have any use of this method, you'd need to map instanceId to screens your self.