Vamp up the README (#446)

closes #443 
The purpose of this PR is to "kick off" better docs and to get one step
closer to getting our docs up on the docs website:

### What the README has now: 
- [x] Quick overview (what this SDK is for, who it's for, some example
use-cases)
- [x] Installation instructions
- [x] Basic usage example (bonus: runnable code)
- [ ] Links to detailed docs over at docs.entropy.xyz (@johnnymatthews
this one is on you)

### BONUS:
- [x] test for the code example (if this ever fails the readme needs to
be updated!)