Added Building the Qt version under MacOSX (using MacPorts)

A section about how to build Subsurface using MacPorts on MacOSX

From 455ecae8c6b035ff885f8820c9d6598089f88602 Mon Sep 17 00:00:00 2001
From: Giuseppe 'ferdy' Miceli <ferdy@ferdy.it>
Date: Wed, 26 Feb 2014 17:09:20 +0100
Subject: [PATCH 2/2] Added Building the Qt version under MacOSX (using
 MacPorts)

Signed-off-by: Giuseppe 'ferdy' Miceli <ferdy@ferdy.it>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>