docs(readme): add Greenkeeper badge

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README.md

@@ -1,5 +1,7 @@
 # angular-fullstack-dockerfile
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/angular-fullstack/angular-fullstack-dockerfile.svg)](https://greenkeeper.io/)
+
 This is the Docker image for the node.js yeoman generator angular-fullstack.
 
 As the generator requires node and several npm modules this image bakes in the dependencies to reduce the time taken to deploy applications.