added method as default arg source

ARCs/arc-0056.md

@@ -178,8 +178,9 @@ interface Method {
        * - global: The data key signifies the global state key to read the value from
        * - local: The data key signifies the local state key to read the value from (for the sender)
        * - literal: the value is a literal and should be passed directly as the argument
+       * - method: The utf8 signature of the method in this contract to call to get the default value. If the method has arguments, they all must have default values. The method **MUST** be readonly so simulate can be used to get the default value.
        */
-      source: "box" | "global" | "local" | "literal";
+      source: "box" | "global" | "local" | "literal" | "method";
     };
   }>;
   /** Information about the method's return value */