Update url-rewriting.rst

Added information about URL rewriting rules for the Hiawatha webserver.
@hsleisink also true.
Personally I'd be fine with just the following as introduction about this webserver:

> Hiawatha <https://www.hiawatha-webserver.org/>`_ is a webserver with a strong focus on security.

More is not needed about the webserver.
Everything else should be about how to get it working with CakePHP...