Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
```Markdown
A `Docker Image` is an ordered collection of root filesystem changes and the corresponding execution parameters for use within the Docker container runtime. Each entity in the collection is referred to as a `layer`.
```

Also, maybe each term should be linkable so when it's referenced (as layer is above) someone can just jump right to the definition.