message: Add more doc
  on why module level variables do not work as singletons
diff: @@ -84,6 +84,9 @@ class Skinner {
  // behaviour with multiple copies of files etc. is erratic at best.
  // XXX: We can still end up with the same file twice in the resulting
  // JS bundle which is nonideal.
  +// See https://derickbailey.com/2016/03/09/creating-a-true-singleton-in-node-js-with-es6-symbols/
  +// or https://nodejs.org/api/modules.html#modules_module_caching_caveats
  +// ("Modules are cached based on their resolved filename")
  if (global.mxSkinner === undefined) {
  global.mxSkinner = new Skinner();
  }
  