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/+/187818Reviewed-by: Katie Hockman <katie@golang.org>

doc/contribute.html

@@ -238,7 +238,8 @@ $ git codereview help
 </pre>
 
 <p>
-prints help text, not an error.
+prints help text, not an error. If it prints an error, make sure that $GOPATH/bin
+is in your $PATH.
 </p>
 
 <p>