Add hint on JSON serialization

lib/pure/marshal.nim

@@ -270,6 +270,7 @@ proc store*[T](s: Stream, data: T) =
 
 proc `$$`*[T](x: T): string =
   ## returns a string representation of `x`.
+  ## Note: to serialize `x` to JSON use $(%x) from the ``json`` module
   var stored = initIntSet()
   var d: T
   shallowCopy(d, x)