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
  * updates the URL to `www.robotstxt.org`, as `robotstxt.org` doesn't
    quite work:

      curl -LsS robotstxt.org
      curl: (7) Failed connect to robotstxt.org:80; Operation timed out

Close h5bp#1487.

CHANGELOG.md

@@ -1,5 +1,7 @@
 ### HEAD
 
+* Add `Disallow:` to `robots.txt`
+  ([#1487](https://github.com/h5bp/html5-boilerplate/issues/1487)).
 * Remove default foreground color from form elements
   ([#1390](https://github.com/h5bp/html5-boilerplate/issues/1390)).
 * Remove default margin from print styles

robots.txt

@@ -1,3 +1,5 @@
-# robotstxt.org/
+# www.robotstxt.org/
 
+# Allow crawling of all content
 User-agent: *
+Disallow: