Add Sacks/Ulam spiral images; content to README.

visualizations/prime_spirals/README.md
	-Add explanation/sample usage of the functions exported by
	`js/script.js`.
	-Add images of either spiral (contained in `img/`).

visualizations/prime_spirals/img/(ulam_spiral, sacks_spiral).png
	-Add images of the Ulam and Sacks spiral, for use in
	`README.md`.