document system requirements for iOS development

QTBUG-38814 is not easily fixed because of Apple policy that one should always use the latest XCode (and it's just a matter of luck if you can still get a new application into the app store despite using less-than-the-latest APIs). That coupled with the fact that XCode 5 is not released for OSX 10.7, means that you need OSX 10.8 or newer to develop iOS apps with the released Qt binaries at this time. It's still possible to build Qt against XCode 4, but the release isn't done that way. So having the release not work on 10.7.5 even though we still "support" Qt on this platform is a surprise for users.