Improve "starting point" for OPA policies

public/content/cookbooks/custom-advisories-opa.md

@@ -28,12 +28,21 @@ With that in mind, the following is a good starting point for fresh policies:
 ```rego
 package policy
 
-default deny := false
-
-default msg := ""
+import future.keywords.contains
+import future.keywords.if
 
 # can be any value that matches one of the valid `advisory_type`s in https://dmd.tanna.dev/schema/#internaladvisorydbschemasql
 default advisory_type := "POLICY"
+
+deny contains msg if {
+	false
+	msg := "This will deny the dependency"
+}
+
+warn contains msg if {
+	false
+	msg := "This will warn about the usage of the dependency"
+}
 ```
 
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