Update README with new minimum MacOSX requirements

Since 70cc8f2 the quartz backend is
using some APIs that are not available on MacOSX 10.4 directly
(i.e. without detecting their availability through dynamic linking).
This means that the quartz backend does not work anymore on MacOSX
10.4 and that the 10.5 SDK (or newer) is needed to build.

Author: Andrea Canciani

README

@@ -121,7 +121,7 @@ Supported, "platform" surface backends
 
 	quartz backend
 	--------------
-	MacOS X >= 10.4 with Xcode >= 2.4
+	MacOS X >= 10.5 with Xcode >= 3.0
 
 	win32 backend
 	-------------