Improve documentation on where to run go2nix

[zagy]

I was trying to update Telegraf and it took me hours to find out that you had to run go2nix from
$GHOME/src/github.com/influxdata/telegraf/cmd/telegraf. Running it in $GHOME/src/github.com/influxdata/telegraf/ results in an empty deps.nix. I never came to my mind that the path could be wrong. So maybe the documentation could hint how to find the actual path in those circumstances. Not knowing Go at all I don't really see the pattern there.

Thanks.

[deliciouslytyped]

I ran into a similar problem, i thought you have to run go get from $PWD/src/.. (but normalized)