message: docs: add guide of function component style
diff: @@ -172,6 +172,28 @@ const app = createApp();
  app.use(Button);
  ```
  +#### 4. Style of Function Components
  +
  +Some components of Vant are provided as function, including `Toast`, `Dialog`, `Notify` and `ImagePreview`. When using function components, `unplugin-vue-components` can not auto import the component style, so we need to import style manually.
  +
  +```js
  +// Toast
  +import { Toast } from 'vant';
  +import 'vant/es/toast/style';
  +
  +// Dialog
  +import { Dialog } from 'vant';
  +import 'vant/es/dialog/style';
  +
  +// Notify
  +import { Notify } from 'vant';
  +import 'vant/es/notify/style';
  +
  +// ImagePreview
  +import { ImagePreview } from 'vant';
  +import 'vant/es/image-preview/style';
  +```
  +
  > Vant supports tree shaking by default, so you don't necessarily need the webpack plugin, if you can't accept the full import of css.
  ### Import all components (not recommended)
  