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