docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # Judge TaskMaster
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/coding-blocks/judge-taskmaster.svg)](https://greenkeeper.io/)
+
 [![Travis](https://img.shields.io/travis/coding-blocks/judge-taskmaster.svg?style=flat-square)](https://travis-ci.org/coding-blocks/judge-taskmaster)
 [![Codecov](https://img.shields.io/codecov/c/github/coding-blocks/judge-taskmaster.svg)](https://codecov.io/gh/coding-blocks/judge-taskmaster)