add pie

README.md

 
 * [Composer Installers](https://github.com/composer/installers) - A  multi-framework Composer library installer.
 * [Composer](https://getcomposer.org/) - A package and dependency manager.
+* [Pie](https://github.com/php/pie) - The official PHP Installer for extensions.
 * [Phive](https://phar.io/) - A PHAR manager.
 * [Pickle](https://github.com/FriendsOfPHP/pickle) - A PHP extension installer.