Adding workerman to the list of event driven libs

Another event driven non-blocking I/O library.
https://github.com/walkor/Workerman

README.md

 * [Ratchet](https://github.com/ratchetphp/Ratchet) - A web socket library.
 * [React](https://github.com/reactphp/react) - An event driven non-blocking I/O library.
 * [Rx.PHP](https://github.com/asm89/Rx.PHP) - A reactive extension library.
+* [Workerman](https://github.com/walkor/Workerman) - An event driven non-blocking I/O library.
 
 ## Logging
 *Libraries for generating and working with log files.*