add installation instructions for pngquant (linux, part of #32)
add installation instructions for pngquant (linux, part of #32)

mihailim/image_optim,toy/image_optim,mihailim/image_optim,ramiroaraujo/image_optim,toy/image_optim,ramiroaraujo/image_optim,weheartit/image_optim,weheartit/image_optim,mihailim/image_optim,weheartit/image_optim,toy/image_optim,ramiroaraujo/image_optim