README: Add a note on build flags.
README: Add a note on build flags.

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