Tour of Jdub

The existing README is great for providing quick examples as briefly as possible, but it can be hard for a newcomer to understand what's going on.

This PR defines a new file with a more detailed explanation of what's going on and how to build your first queries.
This is accurate, but confusing. We have preferred to use `Seq()` or `List()` over `Nil`, since they produce the same thing, but are clearer to read for novices.