Add a short notice to CONTRIBUTING.md
Add a short notice to CONTRIBUTING.md

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