Add maintenance-status badge

Author: williamdes

README.md

@@ -6,6 +6,7 @@
 [![Version](http://poser.pugx.org/wdes/simple-php-model-system/version)](https://packagist.org/packages/wdes/simple-php-model-system)
 [![License](http://poser.pugx.org/wdes/simple-php-model-system/license)](https://packagist.org/packages/wdes/simple-php-model-system)
 [![PHP Version Require](http://poser.pugx.org/wdes/simple-php-model-system/require/php)](https://packagist.org/packages/wdes/simple-php-model-system)
+![maintenance-status](https://img.shields.io/badge/maintenance-passively--maintained-yellowgreen.svg)
 
 A simple PHP model system