Add Podman to Registry doc

What does this Merge Request do and why?

I've been using Podman, and was having trouble running Registry (I had never had to do it before) so I could test Geo replication. I finally found a way to do it and thought it might help others who already use Podman. I don't know if it is unsafe, but I have seen something about rootless containers and it seems I am running the container as root (in the VM Podman is using, as I understand it). I thought since this is a GDK environment that it is ok to just warn about that.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.