update install instructions

Author: troolee

README.md

@@ -85,6 +85,7 @@ Note: You can still use [underscore.js](http://underscorejs.org) (>= 1.7.0) inst
 ## Install
 
 ```html
+<link rel="stylesheet" href="gridstack.css" />
 <script src="gridstack.js"></script>
 <script src="gridstack.jQueryUI.js"></script>
 ```