codereview.cfg: add

Missed this when creating the repo.

Change-Id: Ib30ae8c06a0afc8bb0d67a68d4dc716e66f3dbc8
Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/176458
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
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