igraph vignette start

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/spdep@576 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
igraph vignette start

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/spdep@577 edb9625f-4e0d-4859-8d74-9fd3b1da38cb