veyron/lib/testutil/security: adding a package comment

Change-Id: Ie3b4a0d19edc8e3abaaf2f55677a0e7699ae8960
diff --git a/lib/testutil/security/util.go b/lib/testutil/security/util.go
index 1d8e207..f2a4214 100644
--- a/lib/testutil/security/util.go
+++ b/lib/testutil/security/util.go
@@ -1,3 +1,5 @@
+// Package security contains utility testing functions related to
+// security.
 package security
 
 import (