You can learn more about async at some of these external resources.

* [Cooperative Multitasking for Hack](http://hhvm.com/blog/7091/async-cooperative-multitasking-for-hack)
* [Asio Utilities](https://github.com/hhvm/asio-utilities)
* [HHVM Unit Tests](https://github.com/facebook/hhvm/tree/master/hphp/test/slow/async)

Add external resources section to async
Add external resources section to async
