docs(Checkbox): add direction usage document (#5748)

src/checkbox/README.md

@@ -93,7 +93,6 @@ When Checkboxes are inside a CheckboxGroup, the checked checkboxes's name is an
 <van-checkbox-group v-model="result">
   <van-checkbox name="a">Checkbox a</van-checkbox>
   <van-checkbox name="b">Checkbox b</van-checkbox>
-  <van-checkbox name="c">Checkbox c</van-checkbox>
 </van-checkbox-group>
 ```
 
@@ -107,6 +106,25 @@ export default {
 };
 ```
 
+### Horizontal
+
+```html
+<van-checkbox-group v-model="result" direction="horizontal">
+  <van-checkbox name="a">Checkbox a</van-checkbox>
+  <van-checkbox name="b">Checkbox b</van-checkbox>
+</van-checkbox-group>
+```
+
+```js
+export default {
+  data() {
+    return {
+      result: []
+    };
+  }
+};
+```
+
 ### Maximum amount of checked options
 
 ```html

src/checkbox/README.zh-CN.md

@@ -103,7 +103,6 @@ export default {
 <van-checkbox-group v-model="result">
   <van-checkbox name="a">复选框 a</van-checkbox>
   <van-checkbox name="b">复选框 b</van-checkbox>
-  <van-checkbox name="c">复选框 c</van-checkbox>
 </van-checkbox-group>
 ```
 
@@ -117,6 +116,27 @@ export default {
 };
 ```
 
+### 水平排列
+
+将`direction`属性设置为`horizontal`后，复选框组会变成水平排列
+
+```html
+<van-checkbox-group v-model="result" direction="horizontal">
+  <van-checkbox name="a">复选框 a</van-checkbox>
+  <van-checkbox name="b">复选框 b</van-checkbox>
+</van-checkbox-group>
+```
+
+```js
+export default {
+  data() {
+    return {
+      result: []
+    };
+  }
+};
+```
+
 ### 限制最大可选数
 
 通过`max`属性可以限制复选框组的最大可选数