Add visual reference for hourglasses
Add two illustrations of scales, step 1: Blue Photo Pencil

Reference for this style of sketches:

Our Favorite Tools for Sketching
2009-03-11, by Leah Buley
http://adaptivepath.com/ideas/our-favorite-tools-for-sketching
Add illustrations of scales, step 2: Black Marker
Add illustrations of scales, step 3: Warm Gray Marker
Add illustrations in README