update introduce

docs/introduce.md

@@ -35,11 +35,19 @@
 
 ## 示例
 
+
 ```jsx
 import { Datepicker } from 'antd';
 ReactDOM.render(<Datepicker />, mountNode);
 ```
 
+或者按需加载：
+
+```jsx
+import Datepicker from 'antd/lib/datepicker';
+ReactDOM.render(<Datepicker />, mountNode);
+```
+
 ## 版本
 
 - 稳定版：[![npm package](http://img.shields.io/npm/v/antd.svg?style=flat-square)](https://www.npmjs.org/package/antd)