Document disabling of go modules

[turboMaCk]

I've ran into an issue when trying to use go2nix together with go 1.13 (from this commit). go2nix doesn't work when go modules are on which is usually not problem prior to go 1.13 where modules are off by default. I think that explicitely mentioning this needs to be set to off is only win.

• prevents issue with < go 1.13 in evironments which has this set to on
• works with = go 1.13 by default.