[css-mixins-1] Add Privacy and Security sections

Author: Anders Hartvoll Ruud

css-mixins-1/Overview.bs

@@ -585,4 +585,16 @@ Issue: Should we indeed use {{CSSNestedDeclarations}} for this purpose?
 	should probably not be a regular {{CSSStyleDeclaration}},
 	since only custom properties
 	and the '@function/result' descriptor
-	are relevant.
+	are relevant.
+
+Privacy Considerations {#privacy}
+===============================================
+
+The constructs defined by this specification
+are defined and used entirely within CSS;
+they expose no new information.
+
+Security Considerations {#security}
+===============================================
+
+No issues have been opened against this specification.