docs(readme): add Greenkeeper badge

README.md

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 # fullstack-apollo-react-boilerplate
 
-[![Build Status](https://travis-ci.org/the-road-to-graphql/fullstack-apollo-react-boilerplate.svg?branch=master)](https://travis-ci.org/the-road-to-graphql/fullstack-apollo-react-boilerplate) [![Slack](https://slack-the-road-to-learn-react.wieruch.com/badge.svg)](https://slack-the-road-to-learn-react.wieruch.com/)
+[![Build Status](https://travis-ci.org/the-road-to-graphql/fullstack-apollo-react-boilerplate.svg?branch=master)](https://travis-ci.org/the-road-to-graphql/fullstack-apollo-react-boilerplate) [![Slack](https://slack-the-road-to-learn-react.wieruch.com/badge.svg)](https://slack-the-road-to-learn-react.wieruch.com/) [![Greenkeeper badge](https://badges.greenkeeper.io/the-road-to-graphql/fullstack-apollo-react-boilerplate.svg)](https://greenkeeper.io/)
 
 A full-fledged Apollo Server with Apollo Client starter project with React and Express. [Read more about it in this tutorial to build it yourself](https://www.robinwieruch.de/graphql-apollo-server-tutorial/).