Add example app readme

Author: jevakallio

Example/README.md

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+# React Native App Auth Example
+
+A simple example application for the `react-native-app-auth` library. All relevant JavaScript code
+is in [App.js](App.js).
+
+![Demo](demo.gif)
+
+## Setup
+
+You'll need to have Yarn and the React Native CLI installed:
+
+```sh
+npm install --global yarn react-native-cli
+```
+
+Clone the repo and install dependencies
+
+```sh
+git clone https://github.com/FormidableLabs/react-native-app-auth.git
+cd ./react-native-app-auth/Example
+yarn
+```
+
+## Run
+
+iOS:
+
+```sh
+react-native run-ios
+```
+
+Android:
+
+```
+react-native run-android
+```