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](https://github.com/PurpleBooth/govuk_frontend_toolkit_composer) using [satis](https://getcomposer.org/doc/articles/handling-private-packages-with-satis.md), as [composer](https://getcomposer.org/) 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](https://github.com/PurpleBooth/govuk_frontend_toolkit_composer/tree/master/web).

Feel tree to fork the [repository](https://github.com/PurpleBooth/govuk_frontend_toolkit_composer) with the satis composer repository in. Note that the repository is set to track master on this repository. So it does not require updates when a new version is pushed.

Previous pull request #84