Add Windows instructions on FAQ

[gscardine]

Setting/getting variables on Windows

http://superuser.com/questions/79612/setting-and-getting-windows-environment-variables-from-the-command-prompt

I tried the setx NEWVAR SOMETHING to store the variable permanently, but it wasn't recognized by node as process.env.DATABASE_URL

For now what worked is running set NEWVAR=SOMETHING and the variable is stored just for that process space.

Make sure not to add apostrophes or " to the variable values.