fleshed out documentation a bit

[pdaoust]

Hi there. I knew nothing about installing Gimp plugins, apparently, because it took a while to get this plugin installed. So I added some bits to the documentation to help others in my situation.

(Great plugin, by the way; I did this image tonight. Way easier than dropping 300 images onto the canvas and changing their blending modes one by one.)

[themaninthesuitcase]

Thanks for your changes!