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.
There are two separate queries defined here. One returns a `List`, the other is a `FlatSingleRowQuery` and returns an `Option[User]`.

I have never actually used a `CollectionQuery`, but it seems like that should cover many use cases. Should `SingleRowQuery` and `CollectionQuery` always be preferred to `Query`?  If so, I will follow @emilyst's suggestion and change `GetUsers` to a `CollectionQuery`.