Added comparison with Polymer in README.md
Added comparison with Polymer in README.md

thepian/riot,stonexer/riot,rasata/riot,noodle-learns-programming/riot,txchen/riotjs,xtity/riot,madlordory/riot,qrb/riotjs,scalabl3/riot,laomu1988/riot,crisward/riot,zawsx/js-lib-webcomponents-react-riot,thepian/riot,baysao/riot,noodle-learns-programming/riot,muut/riotjs,laomu1988/riot,duongphuhiep/riot,scalabl3/riot,qrb/riotjs,crisward/riot,dschnare/riot,txchen/riotjs,beni55/riot,duongphuhiep/riot,sylvainpolletvillard/riot,chetanism/riot,AndrewSwerlick/riot,dp-lewis/riot,madlordory/riot,chetanism/riot,dschnare/riot,rthbound/riot,beni55/riot,muut/riotjs,sylvainpolletvillard/riot,AndrewSwerlick/riot,joshcartme/riot,rasata/riot,stonexer/riot,rthbound/riot,xtity/riot,dp-lewis/riot,zawsx/js-lib-webcomponents-react-riot,joshcartme/riot,baysao/riot