Add an illustration for the readme's linear programming example.

[tylerneylon]

There is a nice geometric interpretation of that example that would work well as an image - and not as well spelled out in text.

For my own future notes, the area in (x, y) coordinates is a triangle with corners at (2, 3), (3, 7), and (7, 7). We're minimizing 3x + 2y, so (2, 3) is optimal. Self: see my notebook pages from 445.2014 for more details.