added "docker pulls" badge to readme

README.md

@@ -7,6 +7,7 @@
 [![Join the chat at https://gitter.im/eclipse/ditto](https://badges.gitter.im/eclipse/ditto.svg)](https://gitter.im/eclipse/ditto?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Build Status](https://github.com/eclipse/ditto/workflows/build/badge.svg)](https://github.com/eclipse/ditto/actions?query=workflow%3Abuild)
 [![Maven Central](https://img.shields.io/maven-central/v/org.eclipse.ditto/ditto?label=maven)](https://search.maven.org/search?q=g:org.eclipse.ditto)
+[![Docker pulls](https://img.shields.io/docker/pulls/eclipse/ditto-concierge.svg)](https://hub.docker.com/search?q=eclipse%2Fditto&type=image)
 [![License](https://img.shields.io/badge/License-EPL%202.0-green.svg)](https://opensource.org/licenses/EPL-2.0)
 [![Lines of code](https://img.shields.io/badge/dynamic/xml.svg?label=Lines%20of%20code&url=https%3A%2F%2Fwww.openhub.net%2Fprojects%2Feclipse-ditto.xml%3Fapi_key%3D11ac3aa12a364fd87b461559a7eedcc53e18fb5a4cf1e43e02cb7a615f1f3d4f&query=%2Fresponse%2Fresult%2Fproject%2Fanalysis%2Ftotal_code_lines&colorB=lightgrey)](https://www.openhub.net/p/eclipse-ditto)