Add summary of the different packages (.Contrib, .SqlBuilder, etc.) to main README?

[DylanSp]

I'm wondering if it would be a good idea to add a brief summary of what the different sub-packages (Dapper.Contrib and the like) do to the main README. I didn't even realize they provided additional functionality at first, due to the lack of discoverability. Would this be helpful/appreciated?

[NickCraver]

It would be both helpful and appreciated :)

[RSid]

I'll take a pass at adding these if you don't mind!