docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # AngularUI Router  [![Build Status](https://travis-ci.org/angular-ui/ui-router.svg?branch=master)](https://travis-ci.org/angular-ui/ui-router)
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/angular-ui/ui-router.svg)](https://greenkeeper.io/)
+
 **Note: this is the Angular 1.x source for UI-Router version 1.0.  If you are looking for the source for UI-Router 
 version 0.2.x, it can be found [here](https://github.com/angular-ui/ui-router/tree/legacy)**