Doc/faq.md: added `php -S` way to start a server

In faq.md, I added the `php` command as a way to start a local webserver. I personally think it's a really easy way to start a simple local webserver, and it's build-in for OSX 10.9 or higher.

![1__user_imac-van-arthur-2____documents_sites_urlshort__zsh_](https://cloud.githubusercontent.com/assets/6025224/3733545/2df86186-1709-11e4-9082-f822790d51e1.png)