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.
Oh, I see what you mean, especially following the conversation you referenced. Yeah, I agree: the more the info. can be communicated in a safe- and respectful-feeling way, the better.