Add badge to show number of project installations

Author: SimonFrings

README.md

@@ -1,6 +1,7 @@
 # PromiseTimer
 
 [![CI status](https://github.com/reactphp/promise-timer/workflows/CI/badge.svg)](https://github.com/reactphp/promise-timer/actions)
+[![installs on Packagist](https://img.shields.io/packagist/dt/react/promise-timer?color=blue&label=installs%20on%20Packagist)](https://packagist.org/packages/react/promise-timer)
 
 A trivial implementation of timeouts for `Promise`s, built on top of [ReactPHP](https://reactphp.org/).