Add README documentation

Most of the current documentation is in the form of comments. Comments are great (in fact we need more and better ones), but the template.php file is a big, scary blackbox to most designers, so comments in that are nearly useless to them.

I don’t think we should release a 5.x-1.0 version until we’ve got good docs.

And I also don’t think we should write docs until these issues are completed:

• Source rendering order needs adjustment http://drupal.org/node/121991
• Create starter theme; migrate current style to a sub-theme http://drupal.org/node/171464