Changed vdwDF to vdW-DF and inserted group

git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@11107 c92efa57-630b-4861-b058-cf58834340f0
In light of all the recent vdW-DF variants I removed note about revPRB in the README file

git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@11108 c92efa57-630b-4861-b058-cf58834340f0
Shortening of vdW-DF text

git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@11109 c92efa57-630b-4861-b058-cf58834340f0