Document how to get d-bus working on the Mac

Change-Id: I112afadf00c03ef66109d650f13f950461c2fec1
Reviewed-by: Hanne Linaae <hanne.linaae@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

doc/src/platforms/platform-notes.qdoc

@@ -331,4 +331,12 @@
         }
 
     \endcode
+
+    \section2 D-Bus
+
+    Mac OS X does not ship with a D-Bus daemon, because the operating system has
+    its own IPC mechanisms; therefore Qt binary packages don't usually include
+    the \l{Qt D-Bus} module. However you can install \c dbus via Homebrew, Fink or
+    MacPorts; then when you build Qt it will be detected, and the Qt D-Bus module
+    will be built.
 */