Added details to allow project to be easily included in PHP/Composer projects

We're using this toolkit in a number of symfony projects.

It'd be really useful to be able include this library directly, rather than having to add this as an external package. Once this is merged in, there are two options, either the user can add this repository as a secondary repository in their composer.json, or you can register this repository on [packagist](https://packagist.org/), then the user won't need to add a new repository.

I've noticed for Node.JS/ROR you've chosen to keep the packages separate from this repository, so if you don't want to merge this in on those grounds, you could achieve a similar effect could be achieved by creating a composer.json file in a new repository that contained the following (warning: untested)

```json
{
    "name": "alphagov/govuk_frontend_toolkit_composer",
    "type": "project",
    "description": "A collection of Sass and Javascript tools for generating frontend code.",
    "require": {
        "alphagov/govuk_frontend_toolkit": "dev-master"
    },
    "repositories": [
        {
            "type": "package",
            "package": {
                "name": "alphagov/govuk_frontend_toolkit",
                "version": "master",
                "type": "component",
                "source": {
                    "type": "git",
                    "reference": "master",
                    "url": "https://github.com/alphagov/govuk_frontend_toolkit.git"
                },
                "extra": {
                    "component": {
                        "scripts": [
                            "javascripts/**"
                        ],
                        "styles": [
                            "stylesheets/**"
                        ],
                        "files" : [
                            "images/**"
                        ]
                    }
                },
                "require": {
                    "robloach/component-installer": "*"
                }
            }
        }
    ]
}

```