Document modest image in toolkits

[tacaswell]

See https://pypi.python.org/pypi/ModestImage/0.1 and https://github.com/ChrisBeaumont/mpl-modest-image.

[anntzer]

I think the image refactor of 2.0 made this irrelevant. Suggesting to close this.
Edit: or perhaps not, it still looks faster.