Button add `Multiple Buttons` demo (#3660)

components/button/demo/multiple.md

@@ -0,0 +1,44 @@
+---
+order: 6
+title:
+  zh-CN: 多个按钮组合
+  en-US: Multiple Buttons
+---
+
+## zh-CN
+
+按钮组合使用时，推荐使用1个主操作 + n 个次操作，3个以上操作时把更多操作放到 `Dropdown.Button` 中组合使用。
+
+## en-US
+
+If you need several buttons, we recommend that you use 1 primary button + n secondary buttons, and if there are more than three operations, you can group some of them into `Dropdown.Button`.
+
+
+````jsx
+import { Button, Menu, Dropdown, Icon } from 'antd';
+
+function handleMenuClick(e) {
+  console.log('click', e);
+}
+
+const menu = (
+  <Menu onClick={handleMenuClick}>
+    <Menu.Item key="1">1st item</Menu.Item>
+    <Menu.Item key="2">2nd item</Menu.Item>
+    <Menu.Item key="3">3rd item</Menu.Item>
+  </Menu>
+);
+
+ReactDOM.render(
+  <div>
+    <Button type="primary">primary</Button>
+    <Button type="ghost">secondary</Button>
+    <Dropdown overlay={menu}>
+      <Button type="ghost">
+        more <Icon type="down" />
+      </Button>
+    </Dropdown>
+  </div>,
+  mountNode
+);
+````