sys: add codereview.cfg

See golang.org/cl/4131 for context.

Change-Id: If2ba20a1adf9b40435628753ff7ca77577838813
Reviewed-on: https://go-review.googlesource.com/7760Reviewed-by: David Crawshaw <crawshaw@golang.org>

codereview.cfg

+issuerepo: golang/go