Add github release badge

Author: Dennis Granath

README.md

@@ -1,6 +1,8 @@
 # gocd-plumber
 [![Build Status](https://travis-ci.org/dennisgranath/gocd-plumber.svg?branch=master)](https://travis-ci.org/dennisgranath/gocd-plumber)
 [![license](https://img.shields.io/github/license/mashape/apistatus.svg?maxAge=2592000)](https://github.com/dennisgranath/gocd-plumber/blob/master/LICENSE.md)
+[![Download](https://img.shields.io/github/release/dennisgranath/gocd-plumber.svg)](https://github.com/dennisgranath/gocd-plumber/releases/latest)
+
 
 Create gocd-pipelines from YAML config.