docs: add api reference

Author: michaellee8

README.md

@@ -10,13 +10,10 @@ npm install @toruslabs/react-native-web-browser
 
 ## Usage
 
-```js
-import { multiply } from '@toruslabs/react-native-web-browser';
+Since we implement the exact same API of [`expo-web-browser`](https://docs.expo.dev/versions/latest/sdk/webbrowser/)
+except a few changes the cater Expo-specific stuffs, please refer to the documentation in
+https://docs.expo.dev/versions/v45.0.0/sdk/webbrowser/ for API usages.
 
-// ...
-
-const result = await multiply(3, 7);
-```
 
 ## Contributing