docs(readme): add Packagist downloads badge

Author: sudiptpa

README.md

@@ -5,6 +5,7 @@ Framework-agnostic PHP SDK for PayPal **Webhooks** and legacy **Instant Payment
 [![CI](https://github.com/sudiptpa/paypal-notifications/actions/workflows/ci.yml/badge.svg)](https://github.com/sudiptpa/paypal-notifications/actions/workflows/ci.yml)
 [![Latest Release](https://img.shields.io/github/v/release/sudiptpa/paypal-notifications?sort=semver)](https://github.com/sudiptpa/paypal-notifications/releases)
 [![Packagist](https://img.shields.io/packagist/v/sudiptpa/paypal-notifications)](https://packagist.org/packages/sudiptpa/paypal-notifications)
+[![Downloads](https://img.shields.io/packagist/dt/sudiptpa/paypal-notifications)](https://packagist.org/packages/sudiptpa/paypal-notifications)
 [![PHP](https://img.shields.io/badge/php-8.2--8.5-777bb4.svg)](https://www.php.net/)
 [![License](https://img.shields.io/github/license/sudiptpa/paypal-notifications)](LICENSE)