Update README.md

README.md

 
 * [appserver.io](https://github.com/appserver-io/appserver) - A multithreaded application server for PHP, written in PHP.
 * [php-pm](https://github.com/php-pm/php-pm) - A process manager, supercharger and load balancer for PHP applications.
+* [RoadRunner](https://github.com/spiral/roadrunner) - High-performance PHP application server, load-balancer and process manager.
 
 # Resources
 Various resources, such as books, websites and articles, for improving your PHP development skills and knowledge.