Update README

We require OS X 10.7 and Visual Studio 2010 now.
We only support building with clang on OS X.
The private headers section only applies to Qt 4 builds.

Change-Id: I3c566c6d7a81592f65a2ed6bae9c7185a9d7a3b8
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>