Document usage with nginx instead of apache

I kept getting the following error when I tried to install modules: `Error: Failed to apply catalog: Could not find dependent Service[apache2] for Package[PhpModule_common]`
Until I realised it wasn't so much because of a failed dependancy rather than a wrong notification. Thus I believe it is worth documenting since a lot of people now use nginx instead of apache, and don't want to have to install the later.