updating README for Rails engine instructions
updating README for Rails engine instructions

benlivermore/clearance,zernie/clearance,rubyist/clearance,naveed-ahmad/clearance,janusnic/clearance,crowdtap/clearance,crowdtap/clearance,naveed-ahmad/clearance,openeducation/clearance,thoughtbot/clearance,janusnic/clearance,pedrosmmoreira/clearance,benlivermore/clearance,rubyist/clearance,janusnic/clearance,zernie/clearance,zernie/clearance,naveed-ahmad/clearance,thoughtbot/clearance,pedrosmmoreira/clearance,thoughtbot/clearance,crowdtap/clearance,sferik/clearance,benlivermore/clearance,pedrosmmoreira/clearance