Mention spectate to track changes to mutable data types

Author: jtpio

docs/source/examples/Widget Custom.ipynb

@@ -242,12 +242,12 @@
     "    _model_module = Unicode(module_name).tag(sync=True)\n",
     "    _model_module_version = Unicode(module_version).tag(sync=True)\n",
     "\n",
-   "    _view_name = Unicode('EmailView').tag(sync=True)\n",
-   "    _view_module = Unicode(module_name).tag(sync=True)\n",
-   "    _view_module_version = Unicode(module_version).tag(sync=True)\n",
-   "\n",
-   "    value = Unicode('[email protected]').tag(sync=True)\n",
-   "```"
+    "    _view_name = Unicode('EmailView').tag(sync=True)\n",
+    "    _view_module = Unicode(module_name).tag(sync=True)\n",
+    "    _view_module_version = Unicode(module_version).tag(sync=True)\n",
+    "\n",
+    "    value = Unicode('[email protected]').tag(sync=True)\n",
+    "```"
    ]
   },
   {
@@ -288,6 +288,8 @@
     "Syncing mutable types\n",
     "    \n",
     "Please keep in mind that mutable types will not necessarily be synced when they are modified. For example appending an element to a `list` will not cause the changes to sync. Instead a new list must be created and assigned to the trait for the changes to be synced.\n",
+    "    \n",
+    "An alternative would be to use a third-party library such as [spectate](https://github.com/rmorshea/spectate), which tracks changes to mutable data types.\n",
     "</div>"
    ]
   },