Merge pull request #2 from harukasan/feature/godoc-button

Add Godoc badge.

README.md

@@ -2,6 +2,7 @@ go-libwebp
 ==========
 
 [![Build Status](https://travis-ci.org/harukasan/go-libwebp.svg)](https://travis-ci.org/harukasan/go-libwebp)
+[![GoDoc](https://godoc.org/github.com/harukasan/go-libwebp/webp?status.svg)](https://godoc.org/github.com/harukasan/go-libwebp/webp)
 
 A implementation of Go binding for [libwebp](https://developers.google.com/speed/webp/docs/api) written with cgo.