doc: mention that GOPATH/bin should be in PATH
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 <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>

nawawi/go,CAFxX/go,cloudflare/go,golang/go,akutz/go,golang/go,golang/go,nawawi/go,akutz/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,akutz/go,CAFxX/go,nawawi/go,cloudflare/go,akutz/go,nawawi/go,cloudflare/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,akutz/go,akutz/go,golang/go,cloudflare/go,golang/go