Fixes #7203 - Add CLI guide

installing the hammer-cli-katello RPM (with deps) is trivial. it's configuration part that is a bit more difficult to handle.

for EL6, the process is as follows:
```
yum -y localinstall http://fedorapeople.org/groups/katello/releases/yum/2.0/katello/RHEL/6Server/x86_64/katello-repos-latest.rpm
yum -y localinstall http://yum.theforeman.org/releases/1.6/el6/x86_64/foreman-release.rpm
yum -y localinstall http://mirror.pnl.gov/epel/6/x86_64/epel-release-6-8.noarch.rpm
sudo yum -y install rubygem-hammer_cli_katello
[edit /etc/hammer/cli.modules.d/foreman.yml]
```