Document usage of JS parsers

please provide an example so I could PR to add into readme.

discovered 
```go
func pareJSFile(path string) {
	jsStr, err := ioutil.ReadFile(path)
	if err != nil {
		fmt.Println(err)
		return
	}
	jsAst,err:= js.Parse(parse.NewInputBytes(jsStr))
	if err != nil {
		log.Fatal(err)
	}
	v:= Visitor{}
	js.Walk(v,jsAst)
}
```