I know the kit lens isn't the best for this test (and probably the test isn't perfect either), but I don't have anything better at the moment.
Took 5 shots of the same scene with aperture set to F5.6, F8, F11, F22, F29 - everything else (except shutter speed, of course) was the same.
Picture showing the full image and the 100% crop areas
100% crops, aperture from left to right: F5.6, F8, F11, F22, F29
(for some reason it cannot be linked as picture)
The softness due to diffraction at F22 and F29 is clearly visible when viewing at screen size or on downsized images, too.