message: add docker hub badge
diff: # JHipster Registry
  -[![Build Status][travis-image]][travis-url]
  +[![Build Status][travis-image]][travis-url] [![Docker Pulls](https://img.shields.io/docker/pulls/jhipster/jhipster-registry.svg)](https://hub.docker.com/r/jhipster/jhipster-registry/)
  This is the [JHipster](http://jhipster.github.io/) registry service, based on [Spring Cloud Netflix](http://cloud.spring.io/spring-cloud-netflix/), [Eureka](https://github.com/Netflix/eureka) and [Spring Cloud Config](http://cloud.spring.io/spring-cloud-config/).
  