Added instuction for installing browserstack-cli.
Added instuction for installing browserstack-cli.

bradleylandis/testem,paulot/testem,johanneswuerbach/testem,testem/testem,Addepar/testem,paulot/testem,fotinakis/testem,bradleylandis/testem,johanneswuerbach/testem,paulot/testem,fotinakis/testem,Addepar/testem,Addepar/testem,testem/testem,bradleylandis/testem,johanneswuerbach/testem,testem/testem,fotinakis/testem,Addepar/testem