README: Update Supported Platforms

Be explicit that the supported platforms are only valid for the standalone (non Qt5) packages.
Also update minimum Mac version to 10.6, and simplify Windows versions section.

Change-Id: Ie9d001eec4e6d5dd1ade7d54a1fc75ac0bb4d741
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

README

@@ -4,11 +4,11 @@ Qt Creator is a cross-platform IDE for development with the Qt framework.
 
 Supported Platforms
 ===================
-The binary packages support the following platforms:
+The standalone binary packages support the following platforms:
 
-Windows 7, Windows XP SP2, Windows Vista
-(K)Ubuntu Linux 8.04 (32-bit and 64-bit) or later
-Mac OS 10.5 or later
+Windows XP SP2 or later
+(K)Ubuntu Linux 10.04 (32-bit and 64-bit) or later
+Mac OS 10.6 or later
 
 Building the sources requires Qt 4.8.0 or later.