Add a subsection documenting protocol buffer groups.



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rprotobuf@870 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Add a CITATION file pointing to our JSS preprint on arxiv.org in
addition to the normal auto-generated R package citation().  Also add
a link to the arxiv.org paper in the DESCRIPTION file.



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rprotobuf@871 edb9625f-4e0d-4859-8d74-9fd3b1da38cb