Update README.md

Author: kateinoigakukun

README.md

@@ -4,3 +4,37 @@
 
 Swift framework to interact with JavaScript through WebAssembly.
 
+## Usage
+
+This JavaScript code
+
+```javascript
+const alert = window.alert
+const document = window.document
+
+const divElement = document.createElement("div")
+divElement.innerText = "Hello, world"
+const body = document.body
+body.appendChild(divElement)
+
+alert("JavaScript is running on browser!")
+```
+
+Can be written in Swift using JavaScriptKit
+
+```swift
+import JavaScriptKit
+
+let alert = JSObjectRef.global.alert.function!
+let document = JSObjectRef.global.document.object!
+
+let divElement = document.createElement!("div").object!
+divElement.innerText = "Hello, world"
+let body = document.body.object!
+_ = body.appendChild!(divElement)
+
+alert("Swift is running on browser!")
+```
+
+
+Please see [Example](https://github.com/kateinoigakukun/JavaScriptKit/tree/master/Example) directory for more information