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.
Would you mind altering this to reference http://people.centos.org/arrfab/CentOS7/LiveMedia/RPMS/ instead of pulling from fedora's git?