Added badges for quick access to the dockerhub repo (#18)

README.md

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-[![](https://images.microbadger.com/badges/image/wernight/ngrok.svg)](http://microbadger.com/images/wernight/ngrok "Get your own image badge on microbadger.com") [![Codenvy badge](http://beta.codenvy.com/factory/resources/codenvy-contribute.svg)](http://beta.codenvy.com/f?url=https://github.com/wernight/docker-ngrok 'Start development on Codenvy')
+[![Docker repository](https://img.shields.io/docker/automated/wernight/ngrok.svg)](https://hub.docker.com/r/wernight/ngrok/) [![Build passing](https://img.shields.io/docker/build/wernight/ngrok.svg)](https://hub.docker.com/r/wernight/ngrok/) [![](https://images.microbadger.com/badges/image/wernight/ngrok.svg)](http://microbadger.com/images/wernight/ngrok "Get your own image badge on microbadger.com") [![Codenvy badge](http://beta.codenvy.com/factory/resources/codenvy-contribute.svg)](http://beta.codenvy.com/f?url=https://github.com/wernight/docker-ngrok 'Start development on Codenvy')
 
 A [Docker][docker] image for [ngrok][ngrok] v2, introspected tunnels to localhost.
 It's based on the excellent work of [wizardapps/ngrok][wizardapps/ngrok] and [fnichol/ngrok][fnichol/ngrok].