update install instructions based on #846
update install instructions based on #846

dwhswenson/openpathsampling,openpathsampling/openpathsampling,dwhswenson/openpathsampling,choderalab/openpathsampling,openpathsampling/openpathsampling,dwhswenson/openpathsampling,dwhswenson/openpathsampling,choderalab/openpathsampling,choderalab/openpathsampling,openpathsampling/openpathsampling,openpathsampling/openpathsampling