Add badge to show number of project installations

Author: PaulRotmann

README.md

@@ -1,6 +1,7 @@
 # clue/reactphp-docker
 
 [![CI status](https://github.com/clue/reactphp-docker/workflows/CI/badge.svg)](https://github.com/clue/reactphp-docker/actions)
+[![installs on Packagist](https://img.shields.io/packagist/dt/clue/docker-react?color=blue&label=installs%20on%20Packagist)](https://packagist.org/packages/clue/docker-react)
 
 Async, event-driven access to the [Docker Engine API](https://docs.docker.com/develop/sdk/), built on top of [ReactPHP](https://reactphp.org/).