Added note regarding optional ImageMagick requirement to Showcase
Added note regarding optional ImageMagick requirement to Showcase

vascotenner/holoviews,basnijholt/holoviews,vascotenner/holoviews,ioam/holoviews,ioam/holoviews,ioam/holoviews,basnijholt/holoviews,vascotenner/holoviews,basnijholt/holoviews