Add ZenHub badge

Author: davidmigloz

README.md

@@ -1,6 +1,6 @@
 ![header](https://cloud.githubusercontent.com/assets/6546265/22174630/785cdf04-dfe3-11e6-8cf4-024e8dc1c051.png)
 
-[![GitHub Issues](https://img.shields.io/github/issues/davidmigloz/go-bees.svg)](https://github.com/davidmigloz/go-bees/issues)
+[![ZenHub](https://raw.githubusercontent.com/ZenHubIO/support/master/zenhub-badge.png)](https://zenhub.com)
 [![Build Status](https://travis-ci.org/davidmigloz/go-bees.svg?branch=master)](https://travis-ci.org/davidmigloz/go-bees)
 [![codecov](https://codecov.io/gh/davidmigloz/go-bees/branch/master/graph/badge.svg)](https://codecov.io/gh/davidmigloz/go-bees)
 [![Code Climate](https://codeclimate.com/github/davidmigloz/go-bees/badges/gpa.svg)](https://codeclimate.com/github/davidmigloz/go-bees)