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 definitely hear that. The point here was more to make it clear, in a quick, offhand way, that SQL is a topic that lots of people, even those who aren't formally using databases, can benefit from. The chain of logic that I intended goes like this:

You use spreadsheets to do things that databases do --> SQL is the way to interact with that type of "database" functionality, even without a formal database (e.g., with R and Python's `sqldf` packages) --> Therefore, you should be interested in learning about SQL.

If even mentioning databases is likely to confuse students, though, I'd be up to remove that reference here.