docs(readme): add Greenkeeper badge

https://greenkeeper.io/

Author: greenkeeper[bot]

README.md

@@ -1,4 +1,6 @@
 # AngularJS Full-Stack dependencies
+
+[![Greenkeeper badge](https://badges.greenkeeper.io/angular-fullstack/angular-fullstack-deps.svg)](https://greenkeeper.io/)
 [![Dependency Status](https://david-dm.org/angular-fullstack/angular-fullstack-deps.svg)](https://david-dm.org/angular-fullstack/angular-fullstack-deps)
 [![Dev-Dependency Status](https://david-dm.org/angular-fullstack/angular-fullstack-deps/dev-status.svg)](https://david-dm.org/angular-fullstack/angular-fullstack-deps#info=devDependencies)
 [![Known Vulnerabilities](https://snyk.io/package/npm/angular-fullstack-deps/badge.svg)](https://snyk.io/package/npm/angular-fullstack-deps)