added a text for installation without composer

Author: oscarotero

README.md

@@ -33,6 +33,8 @@ This package is installable and autoloadable via Composer as [embed/embed](https
 $ composer require embed/embed
 ```
 
+If you cannot (or don't want to) use composer, a [PSR-4](https://packagist.org/search/?tags=PSR-4) loader is needed.
+
 ## Usage
 
 ```php