Update README to include SELinux info for RHEL/CentOS 7

[yesrod]

Had to do a bit of digging to figure out why I couldn't get my volume mapping to work under CentOS 7 (I'm new to Docker).

On RHEL7/CentOS 7, if SELinux is enabled, you need to include the ':z' or ':Z' suffix to the volume mapping parameter to tell Docker to apply SELinux labels. The ':z' suffix labels the mounted volume with a shared label that other containers can use; the ':Z' suffix labels the mounted volume with a private label only that container can use.

I ended up finding the info I needed from the docker-run man page, but it might help others if this info is available on the README as well.