Add an explicit public domain dedication statement

The process of dedicating a piece of work under the public domain is not the
same under different legal systems. It is possible that different rights are
given away depending on the origin. Sometimes even the dedication of the work
under public domain is not possible at all. CC0 tries to provide an explicit
way to waive all rights to still provide a secure way for other parties to use
this work [1] and provides a fallback when parts of the license may be judged
invalid under any jurisdiction.

[1] http://creativecommons.org/about/cc0