Updated build instructions for docker base images on CentOS 7

Expanded the README to include detailed build instructions for CentOS 6 and CentOS 7 base images on an CentOS 7 host.

Note: building CentOS 7 on CentOS 7 needs https://github.com/CentOS/sig-cloud-instance-build/pull/15 merged first.
Not sure. I need to test whether that works, but I suspect it won't work for building CentOS 6 on CentOS 7, as I needed to use the rhel6-branch. If I may ask, why do you prefer the RPMs?