Added link to godoc.org in Readme

Added the familiar badge that links to godoc documentation of redigo.

README.markdown

@@ -2,6 +2,7 @@ Redigo
 ======
 
 [![Build Status](https://travis-ci.org/garyburd/redigo.svg?branch=master)](https://travis-ci.org/garyburd/redigo)
+[![GoDoc](https://godoc.org/github.com/garyburd/redigo/redis?status.svg)](https://godoc.org/github.com/garyburd/redigo/redis)
 
 Redigo is a [Go](http://golang.org/) client for the [Redis](http://redis.io/) database.