f backwards compat comment

Author: TheBlueMatt

lightning/src/routing/router.rs

@@ -355,6 +355,8 @@ impl Writeable for RouteParameters {
 		write_tlv_fields!(writer, {
 			(0, self.payment_params, required),
 			(2, self.final_value_msat, required),
+			// LDK versions prior to 0.0.114 had the `final_cltv_expiry_delta` parameter in
+			// `RouteParameters` directly. For compatibility, we write it here.
 			(4, self.payment_params.final_cltv_expiry_delta, required),
 		});
 		Ok(())