Add a recipe for the mallard-snippets package

Mallard [1] is an XML-based markup language for topic-based documentation that is extensively used by the GNOME Documentation Project. The mallard-snippets package provides a complete set of snippets for this language and is based on the latest version of the Mallard 1.0 DRAFT [2] as of Moday, 11 February 2013.

[1] http://www.projectmallard.org/ 
[2] http://projectmallard.org/1.0/index.html