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 actually was thinking about this, too, but then wasn't sure where would be the best place to store the files. Given the way the repo. is structured, is the "slides" directory ok for storing ancillary files, or is there another place that's better?