Add `Disallow:` to robots.txt

See https://github.com/yeoman/generator-mobile/pull/58:

- According to the [robots.txt standard](http://www.robotstxt.org/orig.html) at least one disallow rule must be added: 

       > The record starts with one or more User-agent lines, followed by one or more Disallow lines, as detailed below.

- The URL `robotstxt.org` is broken.
![screenshot 2014-11-02 09 07 39](https://cloud.githubusercontent.com/assets/6025224/4874651/599b3616-6267-11e4-9fc1-8b4d965fecda.png)

See the [discussion about this in h5bp too](https://github.com/h5bp/html5-boilerplate/issues/1487)