Improve README to better summarise the different names and concepts

[uranusjr]

Or introduce some real documentation. It might help to use pip’s implementation as “examples” to how various things can be implemented within the resolver’s model. (e.g. Requires-Python, extras)

[pradyunsg]

@uranusjr Anything needed here now?

[uranusjr]

A full write-up on how the interface should be implemented, why each one is needed, and things the implementer should consider. This will be easier to write when we finish pip’s implementation.