regent: Add documentation on SPMD.
regent: Add documentation on SPMD.

sdalton1/legion,StanfordLegion/legion,StanfordLegion/legion,sdalton1/legion,chuckatkins/legion,chuckatkins/legion,sdalton1/legion,chuckatkins/legion,chuckatkins/legion,sdalton1/legion,StanfordLegion/legion,StanfordLegion/legion,chuckatkins/legion,chuckatkins/legion,StanfordLegion/legion,sdalton1/legion,StanfordLegion/legion,chuckatkins/legion,StanfordLegion/legion,StanfordLegion/legion,sdalton1/legion