Added algorithm reference to README
Added algorithm reference to README

bitliner/keywords-calculator,zelandiya/RAKE-tutorial,Shivraja/keyword-rake,thomasyu888/RAKE,aneesha/RAKE,bitliner/keywords-calculator,scorpiovn/RAKE-tutorial,sfischer13/RAKE,appscluster/RAKE,azhar3339/RAKE-tutorial,bravoman/RAKE-tutorial,pmbaumgartner/RAKE,niima/RAKE