Add a note for old PHP versions

Author: alies-dev

README.md

@@ -17,6 +17,8 @@ You can install the package in to a Laravel app that uses [Nova](https://nova.la
 composer require interaction-design-foundation/nova-unlayer-field
 ```
 
+⚠️ For PHP 7.1 - PHP 7.4 please use [version 0.2.2](https://github.com/InteractionDesignFoundation/nova-unlayer-field/tree/0.2.2) (it uses another namespace).
+
 
 ## Usage