Added a note about plugin assets not working witout url rewrite

http://cakephp.lighthouseapp.com/projects/42880/tickets/25-i-installed-but-dont-see-the-images-and-css-of-debugkit

> Plugin assets do not work without url rewriting, this is in the
documentation for CakePHP itself. I guess it could be repeated for this
and every other plugin.

> I could have sworn it was with the Plugin Assets section, but
apparently its not there. Perhaps its just something I know from
experience.

See also:

* http://cakephp.lighthouseapp.com/projects/42648/tickets/1755-plugin-webroot-without-mod_rewrite-problem
* http://cakephp.lighthouseapp.com/projects/42648-cakephp/tickets/500-plugin-css-directories-bug#ticket-500-8

Signed-off-by: mark_story <mark@mark-story.com>

Refs #GH-1113