Emphasize license-free when discussing license examples

This pull request by @JoshData and I updates the license examples page to explicitly recommend making an agency's data free of licensing, through a worldwide public domain dedication.

This isn't a very radical suggestion - CC0 and the PDDL are already listed on the license examples page. However, CC0 and PDDL are not licenses, but an explicit avoidance of licensing, so I've also updated the list to reflect that, and move CC0 and PDDL to their own section. DHS and the CFPB have both already made some of their work license-free via CC0.

This PR includes a link to the [government data licensing guidance](http://theunitedstates.io/licensing/) that a large number of non-governmental public interest groups signed on to in December. Some of the real world DHS and CFPB examples are referenced there, along with this project itself.