Document how to set ports options via make.conf(5)
This patch is based on:
- https://lists.freebsd.org/pipermail/freebsd-ports-announce/2013-June/000062.html
- current content of /usr/ports/Mk/bsd.options.mk
PR: 194306
Reported by: Daan K. <daanknip__hotmail_com>
MFC after: 2 weeks