playground: add codereview.cfg

See golang.org/cl/4131 for context.

Change-Id: I3bddd792c41677574aa858cbf417b8de7a06bd2e
Reviewed-on: https://go-review.googlesource.com/7753
Reviewed-by: David Crawshaw <crawshaw@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