Feature/revise spec

DO NOT MERGE. FOR DISCUSSION ONLY. @zdne @honzajavorek Please comment.

This PR updates the SPEC based on @zdne and my multiple discussions and refinements.

It also pulls in the current README with some typo fixes. 

@zdne I basically forked and update your branch for this PR into master. Would you prefer I just fork off of master and just create a branch the only adds the spec? Let me know.
Enum is usually values of the type of the enumeration. What we have is closer to a discriminated union that can be used as an enum as well. I think using `union` would be worse. So, `enum` is a compromise of sorts, IMO.