Add issue link.

Author: kevinpschaaf

src/standard/notify-path.html

@@ -488,7 +488,7 @@
       // TODO(kschaaf): This is the path analogue to Polymer.Bind.prepareModel,
       // which provides API for path-based notification on elements with property
       // effects; this should be re-factored along with the Bind lib, either all on
-      // Base or all in Bind.
+      // Base or all in Bind (see issue https://github.com/Polymer/polymer/issues/2547).
       prepareModelNotifyPath: function(model) {
         this.mixin(model, {
           fire: Polymer.Base.fire,