Add defaultSource example

Summary: Closes https://github.com/facebook/react-native/pull/5074

Reviewed By: svcscm

Differential Revision: D2798023

Pulled By: nicklockwood

fb-gh-sync-id: d8aa87bf8dcbecf53b341d5914bbe3354cc753aa

Examples/UIExplorer/ImageExample.js

@@ -187,6 +187,20 @@ exports.examples = [
     },
     platform: 'ios',
   },
+  {
+    title: 'defaultSource',
+    description: 'Show a placeholder image when a network image is loading',
+    render: function() {
+      return (
+        <Image
+          defaultSource={require('./bunny.png')}
+          source={{uri: 'http://facebook.github.io/origami/public/images/birds.jpg'}}
+          style={styles.base}
+        />
+      );
+    },
+    platform: 'ios',
+  },
   {
     title: 'Border Color',
     render: function() {