docs(Cascader): add demos

src/cascader/README.md

@@ -1,7 +1,5 @@
 # Cascader
 
-### Intro
-
 ### Install
 
 ```js
@@ -16,7 +14,64 @@ Vue.use(Cascader);
 ### Basic Usage
 
 ```html
-<van-area title="Title" :area-list="areaList" />
+<van-field
+  is-link
+  readonly
+  label="Area"
+  :value="fieldValue"
+  placeholder="Select Area"
+  @click="show = true"
+/>
+<van-popup v-model="show" round position="bottom">
+  <van-cascader
+    v-model="cascaderValue"
+    title="Select Area"
+    @close="show = false"
+    @finish="onFinish"
+  />
+</van-popup>
+```
+
+```js
+export default {
+  data() {
+    return {
+      show: false,
+      fieldValue: '',
+      cascaderValue: '',
+      options: [
+        {
+          text: 'Zhejiang',
+          value: '330000',
+          children: [{ text: 'Hangzhou', value: '330100' }],
+        },
+        {
+          text: 'Jiangsu',
+          value: '320000',
+          children: [{ text: 'Nanjing', value: '320100' }],
+        },
+      ],
+    };
+  },
+  methods: {
+    onFinish(params) {
+      const { selectedOptions } = params;
+      this.fieldValue = selectedOptions.map((option) => option.text).join('/');
+    },
+  },
+};
+```
+
+### Custom Color
+
+```html
+<van-cascader
+  v-model="cascaderValue"
+  title="Select Area"
+  active-color="#1989fa"
+  @close="show = false"
+  @finish="onFinish"
+/>
 ```
 
 ## API
@@ -24,17 +79,24 @@ Vue.use(Cascader);
 ### Props
 
 | Attribute | Description | Type | Default |
-| --------- | ----------- | ---- | ------- |
-
+| --- | --- | --- | --- |
+| title | Title | _string_ | - |
+| value | Values of selected options | _string \| number_ | - |
+| options | Options | _Option[]_ | `[]` |
+| placeholder | Placeholder of unselected tab | _string_ | `Select` |
+| active-color | Active color | _string_ | `#ee0a24` |
+| closeable | Whether to show close icon | _boolean_ | `true` |
 
 ### Events
 
 | Event | Description | Arguments |
-| ----- | ----------- | --------- |
-
+| --- | --- | --- |
+| change | Emitted when active option changed | `{ value, selectedOptions, tabIndex }` |
+| finish | Emitted when all options is selected | `{ value, selectedOptions, tabIndex }` |
+| close | Emmitted when the close icon is clicked | - |
 
 ### Slots
 
-| Name | Description |
-| ---- | ----------- |
-
+| Name  | Description  |
+| ----- | ------------ |
+| title | Custom title |