added ES6 installation (#259)

* added ES6 installation

* Update README.md

Author: mesqueeb

README.md

@@ -89,6 +89,8 @@ You can then `require` the following modules within your source files:
 ```javascript
 var firebase = require('firebase');
 var firebaseui = require('firebaseui');
+// or for ES6 imports.
+import * as firebaseui from 'firebaseui'
 ```
 
 Or include the required files in your HTML, if your HTTP Server serves the files