Update BUILDING.md
Update BUILDING.md

Updates to the text.
zkoehne/Office-UI-Fabric,kissstudio/Office-UI-Fabric,kissstudio/Office-UI-Fabric,waldekmastykarz/Office-UI-Fabric,waldekmastykarz/Office-UI-Fabric,OfficeDev/Office-UI-Fabric,OfficeDev/Office-UI-Fabric,zkoehne/Office-UI-Fabric