Added info on SNAP package to restrictions

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@12510 f3b2605a-c512-4ea7-a41b-209d697bcdaa
Merge remote branch 'lammps-ro/master' into lammps-icms
Added arXiv link

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@12511 f3b2605a-c512-4ea7-a41b-209d697bcdaa
git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@12512 f3b2605a-c512-4ea7-a41b-209d697bcdaa
Merge remote branch 'lammps-ro/master' into lammps-icms