Add download badge

Author: mnapoli

README.md

@@ -3,6 +3,7 @@
 This package configures Slim 3 to work with the [PHP-DI container](http://php-di.org/).
 
 [![Build Status](https://travis-ci.org/PHP-DI/Slim-Bridge.svg?branch=master)](https://travis-ci.org/PHP-DI/Slim-Bridge)
+[![](https://img.shields.io/packagist/dt/php-di/slim-bridge.svg)](https://packagist.org/packages/php-di/slim-bridge)
 
 The full documentation is here: **http://php-di.org/doc/frameworks/slim.html**