use safer/more aesthetic syntax for setting environment variables on Windows

Created by: cdanielsen

(Cleaned up version of https://github.com/facebookincubator/create-react-app/pull/3530)

Documentation update to fix a maddeningly easy oversite for Windows users. The SET command allows for wrapping the variable assignment expression with double quotes. Using this method negates the need to smash two commands together with the && operator as currently documented.