A few days ago I looked up in the forum and I found out that now it is possible to do a 301 redirection directly with webflow (the idea is that google index your domain only once and doesnt see it as a duplicate).
Maybe this would be available one day, in the mean time, and only with people using GoDaddy (yes.. sorry..) I can show you how I did it. I didn't find the answer here so i'll put it myself.
So this is the way I have configured it so the non-www address redirects to the www version.
Hope this helps.