Update installation docs to include Yarn (#7612)
Update installation docs to include Yarn (#7612)

Add Yarn installation scripts to getting started documentation. With many developers making use of Yarn as of late, providing quick copy/paste scripts will prevent developers from having to type the scripts manually.
grimurd/sequelize,jayprakash1/sequelize,majorleaguesoccer/sequelize,sequelize/sequelize,jayprakash1/sequelize,sequelize/sequelize,mikeringrose/sequelize,nemisj/sequelize,Americas/sequelize,pola88/sequelize,konnecteam/sequelize,inDream/sequelize,sequelize/sequelize,gorangajic/sequelize,Wsiegenthaler/sequelize,p0vidl0/sequelize,konnecteam/sequelize,PhinCo/sequelize,Verdier/sequelize,Americas/sequelize,yonjah/sequelize,inDream/sequelize,Alexsey/sequelize,cbauerme/sequelize,yjwong/sequelize,PsiXdev/sequelize,overlookmotel/sequelize,overlookmotel/sequelize