add link to OpenSeadragon github
add link to OpenSeadragon github

eriksjolund/OpenSeadragonPaperjsOverlay,altert/OpenseadragonFabricjsOverlay,altert/OpenseadragonFabricjsOverlay,TheGuardianWolf/OpenSeadragonPaperjsOverlay,eriksjolund/OpenSeadragonPaperjsOverlay,TheGuardianWolf/OpenSeadragonPaperjsOverlay