增加图片预览的 readme

baseComponents/ImageViewer/readme.md

+```javascript
+import WxImageViewer from 'react-wx-images-viewer';
+class App extends Component {
+
+  state = {
+    imags: [
+      require('./assets/2.jpg'),
+      require('./assets/1.jpg'),
+      require('./assets/0.jpg'),
+      require('./assets/3.jpg'),
+      require('./assets/4.jpg'),
+    ],
+    index: 0,
+    isOpen: false
+  };
+
+  onClose = () =>{
+    this.setState({
+      isOpen: false
+    })
+  }
+
+  openViewer (index){
+    this.setState({
+      index,
+      isOpen: true
+    })
+  }
+
+  render() {
+    const {
+      imags,
+      index,
+      isOpen
+    } = this.state;
+
+    return (
+      <div className="app">
+        <div className="img-list">
+          {/*直接打开*/}
+          <button onClick={this.openViewer.bind(this, 0)}>点击打开图片</button>
+          {
+            this.state.imags.map((item, index) => {
+              return <div className="img" key={item}>
+                <img src={item} alt="" onClick={this.openViewer.bind(this, index)} width="100%" height="auto" className=""/> 
+              </div>
+            })
+          }
+        </div>
+        {
+          isOpen ? <WxImageViewer onClose={this.onClose} urls={this.state.imags} index={index}/> : ""
+        }
+      </div>
+    )
+  }
+}
+
+export default App;
+```