Add comments on compat files

To prevent further confusion.

Bug: 258029505
Test: manual
Change-Id: Iaa145e4480833a224b1a07fc68adb7d3e8a36e4b
diff --git a/private/compat/31.0/31.0.compat.cil b/private/compat/31.0/31.0.compat.cil
index 628abfc..787c92a 100644
--- a/private/compat/31.0/31.0.compat.cil
+++ b/private/compat/31.0/31.0.compat.cil
@@ -1 +1,3 @@
-;; This file can't be empty.
+;; complement CIL file for compatibility between ToT policy and 31.0 vendors.
+;; will be compiled along with other normal policy files, on 31.0 vendors.
+;;