readme updated with default empty tag as function

README.md

@@ -289,7 +289,10 @@ value})``. Possible options are:
   * `normalize` (default: `false`): Trim whitespaces inside text nodes.
   * `explicitRoot` (default: `true`): Set this if you want to get the root
     node in the resulting object.
-  * `emptyTag` (default: `''`): what will the value of empty nodes be.
+  * `emptyTag` (default: `''`): what will the value of empty nodes be. In case
+  you want to use an empty object as a default value, it is better to provide a factory
+   function `() => ({})` instead. Without this function a plain object would
+  become a shared reference across all occurrences with unwanted behavior.
   * `explicitArray` (default: `true`): Always put child nodes in an array if
     true; otherwise an array is created only if there is more than one.
   * `ignoreAttrs` (default: `false`): Ignore all XML attributes and only create