Add Trend demo

src/components/Trend/demo/basic.md

+---
+order: 0
+title: 演示
+---
+
+在数值背后添加一个小图标来标识涨跌情况。
+
+````jsx
+import Trend from 'ant-design-pro/lib/Trend';
+
+ReactDOM.render(
+  <div>
+    <Trend flag="up">12%</Trend>
+    <Trend flag="down" style={{ marginLeft: 8 }}>11%</Trend>
+  </div>
+, mountNode);
+````