Update README
Reminder to myself to testdrive more of readline

Starting to think that this part is really what I want to focus on implementing.
Arrow keys, ctrl+a, ctrl+e, and delete would all be pretty well. Figuring out
how to inject dependencies and provide a sane user interface that is also
testable will be key.
Start implementing a skeleton of Readline(string) (string)

Moved some type declarations out into readline_types.go -- my expectation is that
very little of that will need to be exported.