Addition of Vignettes directory to ensure vignette creation with R-devel

git-svn-id: https://hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/plethy@81883 bc3139a8-67e5-0310-9ffc-ced21a209358
conflicts resolved while setting up Git-SVN bridge

From: Bioconductor Git-SVN Bridge <bioc-sync@bioconductor.org>

git-svn-id: https://hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/plethy@88019 bc3139a8-67e5-0310-9ffc-ced21a209358