FLEX-26703 changed the batch file instructions in README to what actually works in Windows.

Conflicts:
	README
FLEX-26703 Simplifying (and, hopefully, clarifying) instruction for constructFlexForIDE.
Changed 'prepare SDK' section again

After Mihai's effort, I looked into this some more. Turns out the easiest, cross platform and most up-to-date way to do this is using the same script as the Installer. Since I can't find a way to change the FP/AIR versions of an existing SDK that works on Windows, I made that a 'Mac/Linux' only section.

Signed-off-by: Erik de Bruin <erik@ixsoftware.nl>