.: add codereview.cfg

This makes git-codereview review issue references
to refer to the main Go repo.

Change-Id: I17003885f15358f776a475a76a31dac179de9676
Reviewed-on: https://go-review.googlesource.com/33908
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/codereview.cfg b/codereview.cfg
new file mode 100644
index 0000000..3f8b14b
--- /dev/null
+++ b/codereview.cfg
@@ -0,0 +1 @@
+issuerepo: golang/go