(minor) Update README
(minor) Update README

It defaults to installing libgraphicsmagick1-dev anyways, so might as well make that clear.
nagadomi/waifu2x,zyhkz/waifu2x,nagadomi/waifu2x,zyhkz/waifu2x,zyhkz/waifu2x,zyhkz/waifu2x,nagadomi/waifu2x,nagadomi/waifu2x,Spitfire1900/upscaler,nagadomi/waifu2x,Spitfire1900/upscaler