Added slides for introduction to SQL.

Following Issue #559, I've created introduction slides for SQL. The presentation currently runs ~4min30sec, and can be split in half if it needs to be shorter. In comprises an overview of the topic, followed by a short motivational example.
I love that you have the code and data used to generate the slides! I wonder though, if it might be better to put them in separate files so that others can play with them more easily without copying the file contents out. I know that's not much work, but it might be just enough friction to avoid some students having something they dig into.