Specify width and height for Image(remote source)

Summary: Closes https://github.com/facebook/react-native/pull/8486

Differential Revision: D3496850

Pulled By: javache

fbshipit-source-id: f2456b914c420089558484fb87e3d92ed2c923af

docs/Props.md

@@ -22,7 +22,7 @@ class Bananas extends Component {
       uri: 'https://upload.wikimedia.org/wikipedia/commons/d/de/Bananavarieties.jpg'
     };
     return (
-      <Image source={pic} />
+      <Image source={pic} style={{width: 193, height: 110}}/>
     );
   }
 }