Add label for "main registry" of image

[nuest]

What about a label that provides the "main" or "normative" source for an image, e.g. a link to Docker Hub or Quay?

org.label-schema.registry-url = "https://hub.docker.com/_/nginx/"

[lpalgarvio]

I was going to post something about a - "org.label-schema.docker-url".
This seems better

[lpalgarvio]

Created pull request #35