[WIP] Write a README

Goals:

- [x] 10,000ft overview
- [ ] Building & linking instructions
- [ ] Why Traversal? (Especially why `ReducibleType`?)
- [ ] How & when to use `Stream`
- [ ] How & when to use `ReducibleOf`
- [ ] How & when to implement `ReducibleType`
- [ ] How & when to write generically over `ReducibleType`