[new feature] Picker: add default-index prop (#2935)

packages/picker/en-US.md

@@ -31,24 +31,14 @@ export default {
 };
 ```
 
-#### Disable option
+#### Default Index
 
 ```html
-<van-picker :columns="columns" />
-```
-
-```javascript
-export default {
-  data() {
-    return {
-      columns: [
-        { text: 'Delaware', disabled: true },
-        { text: 'Florida' },
-        { text: 'Georqia' }
-      ]
-    };
-  }
-};
+<van-picker
+  :columns="columns"
+  :default-index="2"
+  @change="onChange"
+/>
 ```
 
 #### Show Toolbar
@@ -81,6 +71,26 @@ export default {
 };
 ```
 
+#### Disable option
+
+```html
+<van-picker :columns="columns" />
+```
+
+```javascript
+export default {
+  data() {
+    return {
+      columns: [
+        { text: 'Delaware', disabled: true },
+        { text: 'Florida' },
+        { text: 'Georqia' }
+      ]
+    };
+  }
+};
+```
+
 #### Multi columns
 
 ```html
@@ -137,6 +147,7 @@ When Picker columns data is acquired asynchronously, use `loading` prop to show
 | confirm-button-text | Text of confirm button | `String` | `Confirm` |
 | cancel-button-text | Text of cancel button | `String` | `Cancel` |
 | visible-item-count | Count of visible columns | `Number` | `5` |
+| default-index | Default value index of single column picker | `Number` | `0` |
 
 ### Event
 Picker events will pass different parameters according to the columns are single or multiple