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.
I'm fine with `Seq` instead of `List`. I'm confused about `object` instead of `class`. The existing README uses only case classes and I have always extended `Query` with case classes.