Help new users if their SELinux is not-upgraded yet.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)

if you download the current fedora 20 iso, install then run `yum install docker-io` as per our docs, it will fail - this gives the new users some hint as to what next.