Use a separate license for the userguide

The userguide really describes only the Markdown syntax, so can be
considered to be a derivative work of the original Markdown syntax
description at http://daringfireball.net/projects/markdown/syntax .

Therefore, assuming that the Markdown license at
http://daringfireball.net/projects/markdown/license also applies to the
syntax description, let's publish the userguide with the same license.
Link up the pages with LICENSE.txt in the footer