add CentOS registry links to image readme's

[tchughesiv]

https://github.com/CentOS/container-index/blob/master/index.d/container-examples.yml
e.g.
$ docker pull registry.centos.org/container-examples/starter-arbitrary-uid

[tchughesiv]

systemd done:
https://github.com/RHsyseng/container-rhel-examples/tree/master/starter-systemd#running-in-openshift-w-the-anyuid-scc--root-uid