app.js Update

`
import { 
  createStackNavigator, 
  createSwitchNavigator, 
  createAppContainer 
} from 'react-navigation';

const App = createAppContainer(createSwitchNavigator(
  {
    Loading,
    SignUp,
    Login,
    Main
  },
  {
    initialRouteName: 'Loading'
  }
));
`

I just wanted to add this update of app.js 