readme: added packagist badge

Author: mxschmitt

README.md

@@ -1,6 +1,7 @@
 # SinusBot API PHP Class
 
 [![Build Status](https://travis-ci.org/SinusBot/api-php.svg?branch=master)](https://travis-ci.org/SinusBot/api-php)
+[![Packagist](https://img.shields.io/packagist/v/sinusbot/sinusbot-api-php.svg)](https://packagist.org/packages/sinusbot/sinusbot-api-php)
 
 > PHP Wrapper to communicate with the [SinusBot](https://sinusbot.com) API.