porters-handbook: document move of WWW from pkg-descr to Makefile

Review D36434 will add support for a WWW macro in the ports' Makefiles
instead of at the end of the pkg-descr files to the ports framework.

This will make the WWW entries accessible to more tools (e.g. portgrep)
and will make it easier to maintain them (e.g. to find stale URLs).

Adding WWW: lines at the end of pkg-descr files is deprecated and will
be warned about by a forthcoming commit to portlint.

Follow-up commits will update all ports to conform with this new
style.

Approved by: rene, carlavilla (earlier version)
Differential Revision: https://reviews.freebsd.org/D36369