message: [Doc] swipe: add change event demo
diff: @@ -45,6 +45,27 @@ export default {
  }
  ```
  +#### change event
  +
  +```html
  +<van-swipe @change="onChange">
  + <van-swipe-item>1</van-swipe-item>
  + <van-swipe-item>2</van-swipe-item>
  + <van-swipe-item>3</van-swipe-item>
  + <van-swipe-item>4</van-swipe-item>
  +</van-swipe>
  +```
  +
  +```js
  +export default {
  + methods: {
  + onChange(index) {
  + Toast('Current Swipe index:' + index);
  + }
  + }
  +}
  +```
  +
  ### API
  | Attribute | Description | Type | Default | Accepted Values |
  