Write a README

Goals:

- [x] 10,000ft overview
- [x] Standalone development instructions
- [x] Integration instructions
- [x] Why not `SequenceType`/`GeneratorType`?
- [x] Why Traversal? (Especially why `ReducibleType`?)
- ~~How & when to use `Stream`~~ #22
- ~~How & when to use `ReducibleOf`~~ #22
- ~~How & when to implement `ReducibleType`~~ #23
- ~~How & when to write generically over `ReducibleType`~~ #22
- ~~Reducers~~ #16 
- ~~non-recursive left folds~~ #23

Considering this a good start.