Katie doc edits

[kkowalsky]

Revising the documentation @nvkelso started to fit Mapzen's writing style guide.

Still needs some love, here's the TODO:

• [x] images from @nvkelso of the terrain tiles
• [x] build-a-map.md needs to be finished
• [ ] @rmglennon needs to do writing style checks

[kevinkreiser]

probably didnt mean to check in that .DS_Store file

[zerebubuth]

Great docs, thanks! These will be really useful.

[kkowalsky]

thanks for catching that @kevinkreiser 😅

@zerebubuth IMO I think we can wait on the tutorial and not rush it.