Add README

Author: lorensr

README.md

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+## The GraphQL Guide React Native app
+
+This example app is built, tutorial-style, in the React Native chapter of [The GraphQL Guide](https://graphql.guide/).
+
+- Entry point: [App.js](https://github.com/GraphQLGuide/guide-react-native/blob/master/App.js)
+- Two screens, each with a GraphQL query:
+  - [src/HomeScreen.js](https://github.com/GraphQLGuide/guide-react-native/blob/master/src/HomeScreen.js)
+  - [src/ChapterScreen.js](https://github.com/GraphQLGuide/guide-react-native/blob/master/src/ChapterScreen.js)
+
+### Running the app
+
+```sh
+git clone https://github.com/GraphQLGuide/guide-react-native.git
+cd guide-react-native/
+npm install
+npm start
+```
+
+This will start the Expo bundler, and you can open the app on Android, iOS, or web.
+
+![App running on iOS device](https://res.cloudinary.com/graphql/image/upload/v1601574720/guide/expo-home-screen.png)