doc: add npm3 note in readme
doc: add npm3 note in readme

russjwells/sittingweb,mygithub1216/MERN_SelfCar_Dashboard,OmarBERRABEH/dahakni,zivkaziv/MazorTech,GTDev87/stylizer,zivkaziv/MazorTech,Muisti/muisti,Cathje/KLP,maicongil/fcc_voting_app,carloscuatin/mern-starter,GTDev87/stylizer,denistakeda/evolution,eantler/google,thanhdatvo/tuoihoctro.react,Trulsabe/reactLinuxMern,eantler/simmeme-client,cdriscol/amerikawedding2017,OmarBERRABEH/dahakni,acguanacaste/liomysSalvini,bbviana/alexandria-mern,CODEINAE/todoy,Investnaira/in,b3j0f/lechangement,Investnaira/in,sharvit/my-blog-react,mygithub1216/MERN_SelfCar_Dashboard,yagneshmodh/QuizeApp,Trulsabe/reactLinuxMern,yagneshmodh/QuizeApp,michaelkirschbaum/ender-site,bbviana/alexandria-mern,Hashnode/mern-starter,b3j0f/lechangement,Cathje/KLP,realsoftdev/knack,realsoftdev/knack,sharvit/my-blog-react,ArthurGerbelot/keystamp-mern,carloscuatin/mern-starter,denistakeda/botboard,CODEINAE/todoy,eantler/google,jessicarobins/det,michaelkirschbaum/ender-site,caleb272/PollIt,ArthurGerbelot/keystamp-mern,Muisti/muisti,zivkaziv/MazorTech,Skrpk/mern-sagas,maicongil/fcc_voting_app,denistakeda/evolution,denistakeda/botboard,caleb272/PollIt,acguanacaste/liomysSalvini,Skrpk/mern-sagas,eantler/simmeme-client,thanhdatvo/tuoihoctro.react,jessicarobins/det,Hashnode/mern-starter,russjwells/sittingweb