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.
Bunch of comments here.

 - Either delete the "originating from" language or clarify it: "defined in".
 - Add a colon after "…and two fields"
 - Replace "the actual SQL code" with "the SQL query to run"
 - "When your query class is applied to a database object" -> "When your query is executed"
 - "giving you a chance to do some post-processing…" -> "which produces your final result."