message: update README.md:  add babel-plugin-import
diff: @@ -19,27 +19,40 @@ npm i -S vant
  ## Usage
  +### Use [babel-plugin-import](https://github.com/ant-design/babel-plugin-import) (Recommended)
  +
  + ```js
  + // .babelrc or babel-loader option
  + {
  + "plugins": [
  + ["import", { "libraryName": "vant", "style": true }]
  + ]
  + }
  + ```
  -### Import all components
  + Then you can import components from vant, equivalent to import manually below.
  -```javascript
  -import Vue from 'vue';
  -import vant from 'vant';
  -import 'vant/lib/vant-css/index.css';
  + ```js
  + // import js and css modularly, parsed by babel-plugin-import
  + import { Button } from 'vant';
  + ```
  -Vue.use(vant);
  +### Manually import
  +
  + ```jsx
  + import { Button } from 'vant';
  + import 'vant/lib/vant-css/button.css';
  ```
  -### On demand
  -```javascript
  +### Import all components
  +
  +```javascrip
  import Vue from 'vue';
  -import { Button, Cell } from 'vant';
  -import 'vant/lib/vant-css/button.css';
  -import 'vant/lib/vant-css/cell.css';
  +import vant from 'vant';
  +import 'vant/lib/vant-css/index.css';
  -Vue.component(Button.name, Button);
  -Vue.component(Cell.name, Cell);
  +Vue.use(vant);
  ```
  ## Development
  