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>

README

@@ -8,7 +8,7 @@ The standalone binary packages support the following platforms:
 
 Windows XP SP2 or later
 (K)Ubuntu Linux 11.10 (32-bit and 64-bit) or later
-Mac OS 10.6 or later
+Mac OS X 10.7 or later
 
 Building the sources requires Qt 5.2.0 or later.
 It is also possible to build with Qt 4.8.x, but in that case the Welcome mode,
@@ -20,9 +20,9 @@ Prerequisites:
    * Qt 5.2.0 or later (with restrictions also Qt 4.8.x)
    * On Windows:
      -  ActiveState Active Perl
-     -  MinGW or Visual Studio
+     -  MinGW or Visual Studio 2010 or later
      -  jom
-   * On Mac: Xcode
+   * On Mac OS X: latest Xcode
 
 The installed toolchains have to match the one Qt was compiled with.
 
@@ -32,17 +32,17 @@ You can build Qt Creator with
     qmake -r
     make (or mingw32-make or nmake or jom, depending on your platform)
 
-Note: If you get "make: *** No targets specified and no makefile found. Stop."
-      on Mac OS X, you need to additionally pass "-spec macx-g++" to qmake.
+Note: If you use Qt 4.8 to build on OS X, you need to additionally pass
+      "-spec unsupported/macx-clang-libc++" to qmake.
 
 Installation ("make install") is not needed. It is however possible, using
 
     make install INSTALL_ROOT=$INSTALL_DIRECTORY
 
-Private Header Dependencies
----------------------------
+Private Header Dependencies When Compiling With Qt 4.8
+------------------------------------------------------
 
-The Qt Quick Designer and the QNX plugin, and on Windows the Utils library,use
+The QNX plugin, and on Windows the Utils library, use
 private headers of Qt. These private headers always end with an "_p.h". Digia
 and the Qt Project do not promise to keep these files or APIs binary or source
 compatible between releases. This means that when compiled, Qt Creator has a