Add additional hand-written docs

[samhh]

Including:

• [x] Usage overview for anyone already familiar with the basic idea of sum types
• [ ] Guide for everyone else
• [ ] Deep dive on why we don't support generics
• [ ] How to rename sum type members without missing anything
• [x] Interop with fp-ts pipe

[ericblade]

Agree. Someone pointed out this library to me with a not very well defined purpose as to why I should look at it, and it's not clear at all what purpose it actually fulfills.

[samhh]

@ericblade Does #46 clear things up at all?