Hammer CLI

This PR (https://github.com/theforeman/hammer-cli/pull/251) introduced the ':warning' and ':hidden' options to subcommands, allowing us to show deprecation warnings when a user uses the command (e.g. https://github.com/theforeman/hammer-cli-foreman/blob/master/lib/hammer_cli_foreman.rb#L106). We should have some documentation in the readme to show how this is done rather than relying on developers to search through PRs and grep code to figure it out.