Add some help text into the react-native preview panel

app/react-native/src/manager/components/PreviewHelp.js

@@ -0,0 +1,62 @@
+import PropTypes from 'prop-types';
+import React, { Component } from 'react';
+
+class PreviewHelp extends Component {
+  render() {
+    return (
+      <div style={styles.container}>
+        <p>
+          To see your Storybook stories on the device, you should start your mobile app for the &lt;platform&gt; of your choice (typically ios or android).
+        </p>
+        <div style={styles.instructionsContainer}>
+          <div style={styles.instructions}>
+            <p style={styles.instructionsText}>For create-react-native-app apps:</p>
+            <pre style={styles.instructionsCode}>npm run &lt;platform&gt;</pre>
+          </div>
+          <div style={styles.instructions}>
+            <p style={styles.instructionsText}>For "react-native init" or ejected apps:</p>
+            <pre style={styles.instructionsCode}>react-native run-&lt;platform&gt;</pre>
+          </div>
+        </div>
+      </div>
+    );
+  }
+}
+
+const styles = {
+  container: {
+    flex: 1,
+    display: 'flex',
+    flexDirection: 'column',
+    alignItems: 'center',
+    justifyContent: 'center',
+    background: 'white',
+    width: '100%',
+    height: '100%',
+  },
+
+  instructionsContainer: {
+    display: 'flex',
+    flexDirection: 'row',
+    alignItems: 'center',
+    width: '50%',
+  },
+  instructions: {
+    flex: 1,
+    display: 'flex',
+    flexDirection: 'column',
+    alignItems: 'center',
+    margin: '0 20px',
+  },
+  instructionsText: {
+    marginBottom: '0',
+  },
+  instructionsCode: {
+    width: '100%',
+    padding: '10px',
+    backgroundColor: '#EEEEEE',
+    textAlign: 'center',
+  },
+};
+
+export default PreviewHelp;

app/react-native/src/manager/provider.js

@@ -3,6 +3,7 @@ import { Provider } from '@storybook/ui';
 import createChannel from '@storybook/channel-websocket';
 import addons from '@storybook/addons';
 import uuid from 'uuid';
+import PreviewHelp from './components/PreviewHelp';
 
 export default class ReactProvider extends Provider {
   constructor({ url: domain, options }) {
@@ -47,7 +48,7 @@ export default class ReactProvider extends Provider {
       );
     }
 
-    return innerPreview;
+    return innerPreview || <PreviewHelp />;
   }
 
   handleAPI(api) {