Add Composer installation option

[MattyBalaam]

For some overview: https://groups.google.com/d/msg/silverstripe-dev/kHsCR2llQHQ/mLu_3yJNDuUJ

What needs to be done is creating a Composer.JSON file in the root of your module:

{
    "name": "silverstripe/foo_project_name",
    "description": "foo_description",
    "type": "silverstripe-module",
    "keywords": ["silverstripe", "foo", "bar"],
    "authors": [
    {
        "name": "Foo Bar",
        "email": "foo@bar.com"
    }
    ],

    "require":
    {
        "silverstripe/framework": "3.*"
    }
}

And then submitting to https://packagist.org/packages/silverstripe/

[MrJamesEllis]

Nice one, thanks for the headsup. It's on our todo list.