RCR 203: Make debug mode easier to find

Make it more obvious from command line how to invoke Ruby debugger.

It is currently not obvious how to invoke the Ruby debugger. The output of "ruby -h" does not help either. A new user has to go and look up the correct "-r debug" switch (e.g. in Pickaxe book or elswhere and it is not always easy to find).

A couple of options are:

  <li>Let "-d" parameter be how to invoke the debugger (which has the advantage 
  of being the same as Perl) - in which case, perhaps "-D" could take over from "-d".</li>
  <li>Add some text to output of "ruby -h" to indicate how to run debugger, for example:</li>

  -d  set debugging flags (set $DEBUG to true)
      Note to debug your script use "-rdebug"

This would remove an (unnecessary) stumbling point for newbies.