docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,6 +1,8 @@
 Learning JavaScript Data Structures and Algorithms
 ====================================
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/loiane/javascript-datastructures-algorithms.svg)](https://greenkeeper.io/)
+
 [![Build Status](https://travis-ci.org/loiane/javascript-datastructures-algorithms.svg?branch=third-edition)](https://travis-ci.org/loiane/javascript-datastructures-algorithms)
 [![codecov](https://codecov.io/gh/loiane/javascript-datastructures-algorithms/branch/third-edition/graph/badge.svg)](https://codecov.io/gh/loiane/javascript-datastructures-algorithms)