Add `Disallow:` to `robots.txt`

The addition of `Disallow:` is made in order to be compliant with:

  * the `robots.txt` specification (http://www.robotstxt.org/), which
    specifies that: "At least one Disallow field needs to be present
    in a record"
  * what is suggested in the documentation of most of the major search
    engines, e.g.:

      - Baidu:  http://www.baidu.com/search/robots_english.html
      - Google: https://developers.google.com/webmasters/control-crawl-index/docs/getting_started
                http://www.youtube.com/watch?v=P7GY1fE5JQQ
      - Yandex: help.yandex.com/webmaster/controlling-robot/robots-txt.xml

Besides the addition specified above, this commit also adds a comment making
it clear to everyone that the directives from the `robots.txt` file allow all
content on the site to be crawled.

Ref h5bp/html5-boilerplate#1487.