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'm not worried about confusion, rather making people feel secure in their
spreadsheet skills being "valid."
On Oct 12, 2014 5:48 PM, "publicus" <notifications@github.com> wrote:

> In slides/introduction_to_sql.html:
>
> > +
> > +#######################################
> > +# End of CSV data
> > +#######################################
> > +
> > +-->
> > +
> > +
> > +<section>
> > +  <h1>{{page.title}}</h1>
> > +  <img src="{{page.root}}/img/software-carpentry-banner.png" alt="Software Carpentry logo" />
> > +  <aside class="notes">
> > +    <p>Hello, and welcome to our section on SQL.</p>
> > +    <p>In this section, we're going to be talking about <b>how you use and interact with your data.</b></p>
> > +    <p>Often, researchers might manually move data around, or save different calculations of the same data in different spreadsheets or in different places within the same spreadsheet.
> > +    <p>In these cases, researchers are using spreadsheets, when they often really <b>want to be</b> using "databases."</p>
>
> 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.
>
> --
> Reply to this email directly or view it on GitHub
> <https://github.com/swcarpentry/bc/pull/792/files#r18751673>.
>