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.
Good question. What *are* the relevant use cases for the base `Query` trait?

I've used it to have the `reduce` method return a `Map`, though I could certainly have used a `CollectionQuery` and then done some simple post-processing to produce the same result. Are there other cases where there's a clear benefit to using `Query` directly?