Update the README for current versions of OpenSeadragon

Update the example HTML to match what's here
http://openseadragon.github.io/docs/

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
Update the README to mention VIPS

VIPS can handle much larger files than GraphicsMagick.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>