Add godoc badge

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

Author: LK4D4

README.md

@@ -2,6 +2,7 @@ debugmutex
 ==========
 
 [![Build Status](https://travis-ci.org/LK4D4/debugmutex.svg?branch=master)](https://travis-ci.org/LK4D4/debugmutex)
+[![GoDoc](https://godoc.org/github.com/LK4D4/debugmutex?status.svg)](https://godoc.org/github.com/LK4D4/debugmutex)
 
 Mutex for debugging deadlocks. It can find non-obvious deadlocks in systems
 with heavy `sync.Mutex` usage. I found many deadlocks in Docker with it.