Update to RHEL6 instructions, requires additional channel

To install puppet on RHEL6, an additional channel (repo) is required,
the RHEL6 Optional Channel. Rather than provide explicit directions, as
they have changed a few times over the life of RHEL6, a link is provide
to redhat.com for the viewer to follow.
Merge pull request #271 from rnelson0/rhel-optional-url

Update to RHEL6 instructions, requires additional channel