Please document that ruby is required for the plist script

[gnn at freebsd.org]

Howdy,

On this page:

http://www.freebsd.org/doc/en_US.ISO8859-1/books/porters-handbook/plist-autoplist.html

the plist script is mentioned as a way to get the plist for a port
built somewhat automatically.  What the page does not mention is that
the script in question, /usr/ports/Tools/scripts/plist, requires ruby.

Best,
George