message: [Doc] Tabbar: update demo
diff: @@ -14,9 +14,9 @@ Vue.use(Tabbar).use(TabbarItem);
  ```html
  <van-tabbar v-model="active">
  <van-tabbar-item icon="home-o">Tab</van-tabbar-item>
  - <van-tabbar-item icon="search" dot>Tab</van-tabbar-item>
  - <van-tabbar-item icon="friends-o" info="5">Tab</van-tabbar-item>
  - <van-tabbar-item icon="setting-o" info="20">Tab</van-tabbar-item>
  + <van-tabbar-item icon="search">Tab</van-tabbar-item>
  + <van-tabbar-item icon="friends-o">Tab</van-tabbar-item>
  + <van-tabbar-item icon="setting-o">Tab</van-tabbar-item>
  </van-tabbar>
  ```
  @@ -30,7 +30,18 @@ export default {
  }
  ```
  -#### Custom icon
  +#### Show Badge
  +
  +```html
  +<van-tabbar v-model="active">
  + <van-tabbar-item icon="home-o">Tab</van-tabbar-item>
  + <van-tabbar-item icon="search" dot>Tab</van-tabbar-item>
  + <van-tabbar-item icon="friends-o" info="5">Tab</van-tabbar-item>
  + <van-tabbar-item icon="setting-o" info="20">Tab</van-tabbar-item>
  +</van-tabbar>
  +```
  +
  +#### Custom Icon
  Use `icon` slot to custom icon
  