[improvement] Badge: add change event (#2017)

packages/badge/en-US.md

@@ -14,11 +14,11 @@ Vue.use(BadgeGroup);
 Use `active-key` prop to set index of chosen 'badge'
 
 ```html
-<van-badge-group :active-key="activeKey">
-  <van-badge title="Title" @click="onClick" />
-  <van-badge title="Title" @click="onClick" info="8" />
-  <van-badge title="Title" @click="onClick" info="99" />
-  <van-badge title="Title" @click="onClick" info="199" />
+<van-badge-group :active-key="activeKey" @change="onChange">
+  <van-badge title="Title" />
+  <van-badge title="Title" info="8" />
+  <van-badge title="Title" info="99" />
+  <van-badge title="Title" info="199" />
 </van-badge-group>
 ```
 
@@ -29,8 +29,9 @@ export default {
       activeKey: 0
     };
   },
+
   methods: {
-    onClick(key) {
+    onChange(key) {
       this.activeKey = key;
     }
   }
@@ -43,9 +44,22 @@ export default {
 |------|------|------|------|
 | active-key | Index of chosen badge | `String | Number` | `0` |
 
+### BadgeGroup Event
+
+| Event | Description | Arguments |
+|------|------|------|
+| change | Triggered when badge changed | key: index of current badge |
+
 ### Badge API
+
 | Attribute | Description | Type | Default |
 |------|------|------|------|
 | title | Content | `String` | `''` |
 | info | Info Message | `String | Number` | `''` |
 | url | Link | `String` | - |
+
+### Badge Event
+
+| Event | Description | Arguments |
+|------|------|------|
+| click | Triggered when click badge | key: index of current badge |