docs(readme): add Greenkeeper badge

greenkeeper[bot] · greenkeeper[bot] · commit e5651961f023 · 2017-09-25T13:50:40.000Z
diff --git a/readme.md b/readme.md
@@ -1,4 +1,6 @@
 # xAPI service
+
+[![Greenkeeper badge](https://badges.greenkeeper.io/LearningLocker/xapi-service.svg)](https://greenkeeper.io/)
 > A server for xAPI
 
 [![Build Status](https://travis-ci.org/LearningLocker/xapi-service.svg?branch=master)](https://travis-ci.org/LearningLocker/xapi-service)
