docs(readme): add Greenkeeper badge

https://greenkeeper.io/

README.md

@@ -7,6 +7,8 @@
 
 # [JupyterLab](http://jupyterlab.github.io/jupyterlab/)
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/jupyterlab/jupyterlab.svg)](https://greenkeeper.io/)
+
 [![Build Status](https://travis-ci.org/jupyterlab/jupyterlab.svg?branch=master)](https://travis-ci.org/jupyterlab/jupyterlab)
 [![Binder](http://mybinder.org/badge.svg)](http://mybinder.org/repo/jupyterlab/jupyterlab/lab)
 [![Documentation Status](https://readthedocs.org/projects/jupyterlab-tutorial/badge/?version=latest)](https://jupyterlab-tutorial.readthedocs.io/en/latest/?badge=latest)