Add comment to the OWNERS file.

TBR:perkj@chromium.org
Bug: None
Change-Id: Ief853222fdc8819f78a2a1897eb59f2b324f1d21
diff --git a/OWNERS b/OWNERS
index 0436d0d..1a4160f 100644
--- a/OWNERS
+++ b/OWNERS
@@ -1,2 +1,4 @@
 mbonadei@chromium.org
+
+# Backup reviewer. Prefer reviewers in the list above.
 perkj@chromium.org