message: docs(Picker): add multiple columns demo
diff: @@ -80,6 +80,31 @@ export default {
  };
  ```
  +### Multiple Columns
  +
  +```html
  +<van-picker show-toolbar title="Title" :columns="columns" />
  +```
  +
  +```js
  +export default {
  + data() {
  + return {
  + columns: [
  + {
  + values: ['Monday', 'Tuesday', 'Wednesday', 'Thusday', 'Friday'],
  + defaultIndex: 2
  + },
  + {
  + values: ['Morging', 'Afternoon', 'Evening'],
  + defaultIndex: 1
  + }
  + ]
  + };
  + }
  +};
  +```
  +
  ### Cascade
  ```html
  @@ -261,6 +286,7 @@ Picker events will pass different parameters according to the columns are single
  | values | Value of column | *string[]* |
  | defaultIndex | Default value index | *number* |
  | className | ClassName for this column | *any* |
  +| children | Cascade children | *Column* |
  ### Methods
  