update docs usage

Author: huangminlinux

README.md

@@ -14,9 +14,10 @@ react-native link
 
   > your component.js
 
-  ```
+  ```javascript
   ...
   import JAnalyticsModule from 'janalytics-react-native';
+  JAnalyticsModule.setup({appKey: "you appKey get from jiguang website"})  // iOS 端需要先调用该方法
   ```
 
 - You can refer to [example](./example)