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 would claim that the existing example is non-idiomatic jdub code. It returns a single row, so should use one of the `SingleRowQuery` traits.  This change gives a wider understanding of what jdub can do, but increases the length of the README.