docs: add demo for `overflowCount`

components/badge/demo/overflow.md

@@ -0,0 +1,15 @@
+# 封顶数字
+
+- order: 6
+
+超过 `overflowCount` 的会显示为 `${overflowCount}+`。
+
+---
+
+````jsx
+import { Badge } from 'antd';
+
+ReactDOM.render(<Badge count={99} overflowCount={10}>
+  <a href="#" className="head-example"></a>
+</Badge>, document.getElementById('components-badge-demo-overflow'));
+````