docs: add tree demo

Author: tangjinzhou

components/tree/demo/index.vue

@@ -8,6 +8,7 @@ import Line from './line';
 import Search from './search';
 import Directory from './directory';
 import ReplaceFields from './replaceFields';
+import SwitcherIcon from './switcher-icon';
 
 import CN from '../index.zh-CN.md';
 import US from '../index.en-US.md';
@@ -45,6 +46,7 @@ export default {
         <Line />
         <Search />
         <Directory />
+        <SwitcherIcon />
         <api>
           <template slot="cn">
             <CN />

components/tree/demo/switcher-icon.md

@@ -0,0 +1,44 @@
+<cn>
+#### 自定义展开/折叠图标
+自定义展开/折叠图标。
+</cn>
+
+<us>
+#### Customize collapse/expand icon
+customize collapse/expand icon of tree node
+</us>
+
+```tpl
+<template>
+  <a-tree
+    showLine
+    :defaultExpandedKeys="['0-0-0']"
+    @select="onSelect"
+  >
+    <a-icon slot="switcherIcon" type="down" />
+    <a-tree-node title="parent 1" key="0-0">
+      <a-tree-node title="parent 1-0" key="0-0-0">
+        <a-tree-node title="leaf" key="0-0-0-0" />
+        <a-tree-node title="leaf" key="0-0-0-1" />
+        <a-tree-node title="leaf" key="0-0-0-2" />
+      </a-tree-node>
+      <a-tree-node title="parent 1-1" key="0-0-1">
+        <a-tree-node title="leaf" key="0-0-1-0" />
+      </a-tree-node>
+      <a-tree-node title="parent 1-2" key="0-0-2">
+        <a-tree-node title="leaf" key="0-0-2-0" />
+        <a-tree-node title="leaf" key="0-0-2-1" />
+      </a-tree-node>
+    </a-tree-node>
+  </a-tree>
+</template>
+<script>
+  export default {
+    methods: {
+      onSelect(selectedKeys, info) {
+        console.log('selected', selectedKeys, info);
+      },
+    },
+  };
+</script>
+```