Added composer (PHP) section.

Hi @bradleywright 

As requested here's the PHP setup for composer. 

To get it working I had to setup a custom repository using [satis](https://getcomposer.org/doc/articles/handling-private-packages-with-satis.md), as composer doesn't play nice with submodules. I've taken the liberty of hosting it [here](https://govuk-frontend-toolkit.appspot.com/index.html), however if you want you can take charge of doing that yourself (just make the document root the "web/" directory).