update README
update README

updated the README with some notes on git

jcupitt/libvips,jcupitt/libvips,mcanthony/libvips,leiyangyou/libvips,mcanthony/libvips,mcanthony/libvips,lovell/libvips,leiyangyou/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,wishabi/libvips,wishabi/libvips,jcupitt/libvips,leiyangyou/libvips,mcanthony/libvips,wishabi/libvips,leiyangyou/libvips,lovell/libvips,wishabi/libvips