Add example app build that uses expo

[Chris Brody]

I found a nice guide: https://blog.callstack.io/adding-an-example-app-to-your-react-native-library-d23b9741a19c

The expo demo app should have a simple index.js or App.js file that imports and uses example/AllStories.js, in a similar fashion to example/native/index.js and example/web/example.js.

This would really help me as I had trouble with the native build of the example app on both Android and iOS.

I hope to contribute this in the near future.