Added a note about libpng, libjpeg and ghostscript dependencies for GraphicsMagick
Added a note about libpng, libjpeg and ghostscript dependencies for GraphicsMagick

nicolaasmatthijs/Hilary,Orodan/Hilary,stuartf/Hilary,Coenego/Hilary,timdegroote/Hilary,nicolaasmatthijs/Hilary,simong/Hilary,Orodan/Hilary,mrvisser/Hilary,simong/Hilary,stuartf/Hilary,ExC0d3/Hilary,timdegroote/Hilary,jfederico/Hilary,ExC0d3/Hilary,mrvisser/Hilary,jfederico/Hilary