chore: add an example for SSR (#8298)

<img width="740" alt="Screen Shot 2020-05-20 at 16 31 30" src="https://user-images.githubusercontent.com/1174278/82458770-673d8880-9ab7-11ea-81d3-8ac0c1e52705.png">

example/App.tsx

@@ -1,6 +1,10 @@
 import 'react-native-gesture-handler';
 import { registerRootComponent } from 'expo';
+import { Asset } from 'expo-asset';
+import { Assets as StackAssets } from '@react-navigation/stack';
 
 import App from './src/index';
 
+Asset.loadAsync(StackAssets);
+
 registerRootComponent(App);