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.
Normally we'd use `object` here. I also prefer `Seq` to `List`, since it specifies an interface instead of an implementation. (Also, `Seq` is way, way better than `List`.)