Documentation

[FintanH]

I think it's time we start to add documentation and example usage to the different directories. It'd be a lot of work so it can split up into PR chunks if one wishes and we can track them with the checklist below:

• [ ] Applicative
• [ ] Codensity
• [ ] Compose
• [ ] Either
• [ ] EitherT
• [ ] Foldable
• [ ] Functor
• [ ] Id
• [ ] List
• [ ] Monad
• [ ] Monoid
• [ ] NonEmptyList
• [ ] Optional
• [ ] Reader
• [ ] ReaderT
• [ ] Semigroup
• [ ] State
• [ ] StateT
• [ ] Transformer
• [ ] Traversable
• [ ] Yoneda