Introduction post for product blog

This is my first attempt at an introductory post for our FOIA product blog. It wouldn't be out of place on 18F's [main blog](http://18fblog.tumblr.com), and probably has some overlap with the post @rjmajma has been working on there. But my goals here are to:

* Introduce the blog, and the repository.
* Communicate the current state of our project plans (as we've communicated internally, and to the task force, and to the rest of 18F) to the rest of the world.

Please use this PR to give feedback on how well it's achieving those, and to make non-controversial or consensus-gained commits on the `intro` branch directly, pre-merge.

In general, I'm hoping we'll all feel free to post as frequently and as technically as we want, without having to schedule time on the 18F blog, or worry about alienating less technical readers. Also, not all posts will be technical -- this one clearly isn't, and instead it serves as a development marker and something we can point to for others.

I'll also add that I've become a big fan of the [open, narrative pull request](https://github.com/18F/foia/pull/4). I'm hoping we'll do a bunch of those, with a positive incentive that some PRs could be quickly adapted into blog posts.

This is on a `gh-pages` branch, because I'm intending this to start out at `https://18f.github.io/foia`. Certainly possible that this changes over time. I don't have an actual Jekyll site, much less a theme, ready, but that will be next and can be set up in parallel to actually approving the content.

Possibly to do before publishing:

- [ ] Add a link to a yet-to-be-created issue/PR around an API specification.
- [ ] Add a link to our yet-to-be-published 18F blog post about our open source policy.

But those can also be added post-publication. After all, any changes to posts will be transparent and visible right in this repo!