Added a reference to whoops-stackphp

Hi!

I just built a very simple bridge between [StackPHP](http://stackphp.com/) and Whoops (https://github.com/thecodingmachine/whoops-stackphp). If you are not aware, StackPHP is a development convention based on Symfony 2 HttpKernelInterface to build "middlewares" routers that can be chained easily.
The Whoops middleware is heavily based on the code of the Silex integration, and is now used by default in the Mouf PHP framework (http://mouf-php.com)
Hope you'll enjoy it.