Merge pull request #214 from SimonFrings/installations

[3.x] Add badge to show number of project installations

Author: WyriHaximus

README.md

@@ -5,6 +5,7 @@ A lightweight implementation of
 [CommonJS Promises/A](http://wiki.commonjs.org/wiki/Promises/A) for PHP.
 
 [![CI status](https://github.com/reactphp/promise/workflows/CI/badge.svg)](https://github.com/reactphp/promise/actions)
+[![installs on Packagist](https://img.shields.io/packagist/dt/react/promise?color=blue&label=installs%20on%20Packagist)](https://packagist.org/packages/react/promise)
 
 > **Development version:** This branch contains the code for the upcoming 3.0 release.
 > For the code of the current stable 2.x release, check out the