add expoUsePushy demo (#495)

* add expoUsePushy demo

* update

Example/expoUsePushy/index.js

@@ -0,0 +1,8 @@
+import { registerRootComponent } from 'expo';
+
+import App from './App';
+
+// registerRootComponent calls AppRegistry.registerComponent('main', () => App);
+// It also ensures that whether you load the app in Expo Go or in a native build,
+// the environment is set up appropriately
+registerRootComponent(App);