quickstart: updating document

updating document quickstart.md. add ‘Notification’ to Component list.

examples/docs/en-US/quickstart.md

@@ -259,7 +259,8 @@ import {
   ColorPicker,
   Loading,
   MessageBox,
-  Message
+  Message,
+  Notification
 } from 'element-ui'
 
 Vue.use(Pagination)