Improve the project readme

[Keavon]

The readme contains some out of date information and lots of extraneous details that should be rewritten so that it's much more approachable. It should contain a roadmap and a clear and enlightening product description. A summarized version would also be helpful for a simple landing page for www.graphite.design.

• [x] First pass (around 0.1 release)
• [ ] Second pass (a few months later)