Add recommendation that fio be run in a native console on Windows.

Add recommendation that fio be run in a native console on Windows.

Windows console applications often don't run correctly under Cygwin shells such
as mintty because of the way they're implemented. Add a recommendation to the
README to run fio under a native console such as the Command Prompt or console2
to avoid these problems.

http://code.google.com/p/mintty/issues/detail?id=56 documents the problem.

Signed-off-by: Jens Axboe <axboe@kernel.dk>