Add Prepd app to showcase

Summary:
This app is written entirely in React Native, and available on the iOS and Google Play app stores.

Blogpost: https://logbook.hanno.co/prepd-pros-cons-react-native/
Case study: https://hanno.co/work/prepd/
Closes https://github.com/facebook/react-native/pull/12434

Differential Revision: D4601147

Pulled By: hramos

fbshipit-source-id: b00ddc4d99574405c2c78a9e89e41d4ebc352f1c

website/src/react-native/showcase.js

@@ -369,6 +369,14 @@ var featured = [
     infoLink: 'http://x.co/FlareNews',
     infoTitle: 'Social network that connects entrepreneurs to fellow entrepreneurs, consumers, investors and experts',
   },
+  {
+    name: 'Prepd',
+    icon: 'http://is4.mzstatic.com/image/thumb/Purple111/v4/03/56/95/035695ab-77dc-4f3b-0961-3263e32e3494/source/175x175bb.jpg',
+    linkAppStore: 'https://itunes.apple.com/us/app/get-prepd/id1125687122?mt=8',
+    linkPlayStore: 'https://play.google.com/store/apps/details?id=com.prepd&hl=en',
+    infoLink: 'https://logbook.hanno.co/prepd-pros-cons-react-native/',
+    infoTitle: 'How we built Prepd for iOS and Android using React Native',
+  },
 ];
 
 /*