Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
I think this ought to be more definitive here. Also as I understand it, a layer is always comprised of both the layer json and the fs changes (which may be empty).

```Markdown
Images are a collection of `layers`. A `layer` is a general term which is comprised of the following:
<ol>
    <li>JSON metadata describing the execution parameters used to generate this layer</li>
    <li>The filesystem changes that occurred as a result of the described execution parameters</li>
</ol>
```