Update `robots.txt` file

* Update the URL to `www.robotstxt.org`.
* Add comment making it clear to everyone that the directives from
  the `robots.txt` file allow all content on the site to be crawled.
* Add `Disallow:` directive.

Ref: https://github.com/h5bp/html5-boilerplate/commit/4e5f438e1ea0d7e39179ba5cf799a8d326ba7cc3
Merge pull request #81 from alrra/update-robotstxt

Update `robots.txt` file