docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -25,6 +25,8 @@
 
 ------
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/fs-opensource/hapi-dev-errors.svg)](https://greenkeeper.io/)
+
 <p align="center"><sup>Development of this hapi plugin is supported by <a href="https://futurestud.io">Future Studio University 🚀</a></sup>
 <br><b>
 Join the <a href="https://futurestud.io/university">Future Studio University and Skyrocket in Node.js</a></b>