docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,6 +1,6 @@
 # react-intersection-observer-api-example
 
-[![Build Status](https://travis-ci.org/the-road-to-learn-react/react-intersection-observer-api-example.svg?branch=master)](https://travis-ci.org/the-road-to-learn-react/react-intersection-observer-api-example) [![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-learn-react/react-intersection-observer-api-example.svg?branch=master)](https://travis-ci.org/the-road-to-learn-react/react-intersection-observer-api-example) [![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-learn-react/react-intersection-observer-api-example.svg)](https://greenkeeper.io/)
 
 A simple application that showcases how [Intersection Observer API](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) can be used in React with [createRef()](https://reactjs.org/docs/refs-and-the-dom.html). Use case: observe sections in an article and track the current viewed section in a navigation. [Read more about it](https://www.robinwieruch.de/react-intersection-observer-api).