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.
Null values do not return `None`, unless there is some magic in the flat query stuff. Even if that is the case, I'd be wary of making this statement without qualifications, since it is definitely not true in every circumstance.