doc: mention that GOPATH/bin should be in PATH

As noticed while mentoring at Gophercon, a common mistake for
beginners is that, after they install git-codereview, it's not
found by git because they haven't put GOPATH/bin in their
environment.

Change-Id: I885cee0f532a4069d1a17ea27365d54965ecca22
Reviewed-on: https://go-review.googlesource.com/c/go/+/187818


Reviewed-by: Katie Hockman <katie@golang.org>