Add `Disallow:` to `robots.txt`

According to the [robots.txt standard](http://www.robotstxt.org/orig.html):

> The record starts with one or more User-agent lines, followed by one or more Disallow lines, as detailed below.

Also, [Google seems to recommend it](https://www.youtube.com/watch?v=P7GY1fE5JQQ).

See the [discussion about this in h5bp too](https://github.com/h5bp/html5-boilerplate/issues/1487)