README: add link to godoc.org

Author: pmezard

README.md

@@ -2,6 +2,7 @@ go-difflib
 ==========
 
 [![Build Status](https://travis-ci.org/pmezard/go-difflib.png?branch=master)](https://travis-ci.org/pmezard/go-difflib)
+[![GoDoc](https://godoc.org/github.com/pmezard/go-difflib/difflib?status.svg)](https://godoc.org/github.com/pmezard/go-difflib/difflib)
 
 Go-difflib is a partial port of python 3 difflib package. Its main goal
 was to make unified and context diff available in pure Go, mostly for