Add example about get response body outside the callback function

[typenoob]

package main

import (
    "bytes"
    "fmt"
    curl "github.com/andelf/go-curl"
)

func main() {
    easy := curl.EasyInit()
    defer easy.Cleanup()

    easy.Setopt(curl.OPT_URL, "https://www.baidu.com/")

    buf = new(bytes.Buffer)

    // OPTIONAL - make a callback function
    fooTest := func (ptr []byte, userdata interface{}) bool {
        buf.Write(ptr)
        return true
    }

    easy.Setopt(curl.OPT_WRITEFUNCTION, fooTest)

    if err := easy.Perform(); err != nil {
        fmt.Printf("ERROR: %v\n", err)
    } else {
       println("DEBUG: size=>", buf.Len())
       println("DEBUG: content=>", buf.String())
    }
}

What do you think? Can this helps others?