Update centos.md
Update centos.md

Added instructions for the workaround for CentOS minimal installation where the Docker daemon cannot start due to a conflict with the CentOS firewall

Signed-off-by: Brian Christner <f90a8e7c6bee4d53a9f25d71018df438ecd5ef86@gmail.com>

vegasbrianc/docker,vegasbrianc/docker,vegasbrianc/docker