iOS: Clarify Xcode 5 requirement for developing with Qt for iOS

Change-Id: I3472fb011f9e669efe5b8b8a7b90bfb662063688
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>