Add polypill comment to App.js

Author: tanx

mobile/App.js

@@ -1,5 +1,8 @@
 import 'node-libs-react-native/globals';
 import { Platform } from 'react-native';
+
+// Polyfill ECMAScript Internationalization API on Android
+// See: https://github.com/facebook/react-native/issues/19410
 if (Platform.OS === 'android') {
   require('intl');
   require('intl/locale-data/jsonp/en-US');