Added OpenMage to CMS

See https://www.openmage.org/

README.md

 * [Kirby](https://getkirby.com/) - A flat-file CMS that adapts to any project.
 * [Magento](https://github.com/magento/magento2) - The most popular e-commerce platform.
 * [Moodle](https://moodle.org/) - An open-source learning platform.
+* [OpenMage](https://github.com/OpenMage/magento-lts) - Fork of EoL Magento 1 e-commerce platform.
 * [Pico CMS](https://picocms.org/) - A stupidly simple, blazing fast, flat file CMS.
 * [Statamic](https://statamic.com/) - Build beautiful, easy-to-manage websites.
 * [Sulu](https://sulu.io/) - A user and developer friendly focused CMS and Platform based on the Symfony Framework.