docs: badge for supported PHP versions

Signed-off-by: Tom Carrio <[email protected]>

Author: tcarrio

README.md

@@ -4,6 +4,7 @@
 [![Specification](https://img.shields.io/static/v1?label=Specification&message=v0.5.0&color=yellow)](https://github.com/open-feature/spec/tree/v0.5.0)
 [![Latest Stable Version](http://poser.pugx.org/open-feature/sdk/v)](https://packagist.org/packages/open-feature/sdk)
 [![Total Downloads](http://poser.pugx.org/open-feature/sdk/downloads)](https://packagist.org/packages/open-feature/sdk)
+![PHP 7.4+](https://img.shields.io/badge/php->=7.4-blue.svg)
 [![License](http://poser.pugx.org/open-feature/sdk/license)](https://packagist.org/packages/open-feature/sdk)
 
 ## Overview