Add phpstan badge to readme file

Author: Mohammadreza-73

README.md

@@ -5,6 +5,7 @@
     <a href="https://packagist.org/packages/m.rahimi/astro-orm"><img src="https://img.shields.io/packagist/v/m.rahimi/astro-orm" alt="Latest Stable Version"></a>
     <a href="https://github.styleci.io/repos/433058724?branch=master"><img src="https://github.styleci.io/repos/433058724/shield?branch=master" alt="StyleCI"></a>
     <a href="https://packagist.org/packages/m.rahimi/astro-orm"><img src="https://img.shields.io/packagist/l/m.rahimi/astro-orm" alt="License"></a>
+    <a href="https://phpstan.org/"><img src="https://img.shields.io/badge/PHPStan-level%206-brightgreen.svg?style=flat" alt="PHPStan"></a>
 </p>
 
 object-relational mapper (ORM), lets you query and manipulate data with fluent api from a database using an object-oriented paradigm.