Add usage exemple for conman generator

generators/puppet/conmang5k.rb

@@ -24,6 +24,7 @@ OptionParser.new do |opts|
 
   opts.separator ""
   opts.separator "Example: ruby conmang5k.rb -s nancy -o /tmp/puppet-repo"
+  opts.separator "Example: ruby conmang5k.rb -s nancy -o $GPUPPET_PREFIX/repo"
 
   opts.on('-o', '--output-dir dir', String, 'Select the puppet repo path', "Default: " + options[:output_dir]) do |d|
     options[:output_dir] = d