add DynamicState to FAQ (#146)

Author: aleneum

Diff for: examples/Frequently asked questions.ipynb

@@ -301,11 +301,65 @@
   {
    "cell_type": "markdown",
    "metadata": {},
+   "source": [
+    "You lose `model.is_<state>` convenience functions and the ability to add callbacks such as `Model.on_enter_<state>` automatically. However, the second limitation can be tackled with dynamic resolution in states as pointed out by [mvanderlee](https://github.com/mvanderlee) [here](https://github.com/pytransitions/transitions/issues/146#issuecomment-869049925):"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "outputs": [],
+   "source": [
+    "from transitions import State\n",
+    "import logging\n",
+    "\n",
+    "logger = logging.getLogger(__name__)\n",
+    "\n",
+    "\n",
+    "class DynamicState(State):\n",
+    "    \"\"\" Need to dynamically get the on_enter and on_exit callbacks since the\n",
+    "        model can not be registered to the Machine due to Memory limitations\n",
+    "    \"\"\"\n",
+    "\n",
+    "    def enter(self, event_data):\n",
+    "        \"\"\" Triggered when a state is entered. \"\"\"\n",
+    "        logger.debug(\"%sEntering state %s. Processing callbacks...\", event_data.machine.name, self.name)\n",
+    "        if hasattr(event_data.model, f'on_enter_{self.name}'):\n",
+    "            event_data.machine.callbacks([getattr(event_data.model, f'on_enter_{self.name}')], event_data)\n",
+    "        logger.info(\"%sFinished processing state %s enter callbacks.\", event_data.machine.name, self.name)\n",
+    "\n",
+    "    def exit(self, event_data):\n",
+    "        \"\"\" Triggered when a state is exited. \"\"\"\n",
+    "        logger.debug(\"%sExiting state %s. Processing callbacks...\", event_data.machine.name, self.name)\n",
+    "        if hasattr(event_data.model, f'on_exit_{self.name}'):\n",
+    "            event_data.machine.callbacks([getattr(event_data.model, f'on_exit_{self.name}')], event_data)\n",
+    "        logger.info(\"%sFinished processing state %s exit callbacks.\", event_data.machine.name, self.name)\n",
+    "\n",
+    "\n",
+    "class DynamicMachine(Machine):\n",
+    "    \"\"\"Required to use DynamicState\"\"\"\n",
+    "    state_cls = DynamicState"
+   ],
+   "metadata": {
+    "collapsed": false,
+    "pycharm": {
+     "name": "#%%\n"
+    }
+   }
+  },
+  {
+   "cell_type": "markdown",
    "source": [
     "### Is there a 'during' callback which is called when no transition has been successful?\n",
     "\n",
     "Currently, `transitions` has no such callback. This example from the issue discussed [here](https://github.com/pytransitions/transitions/issues/342) might give you a basic idea about how to extend `Machine` with such a feature:"
-   ]
+   ],
+   "metadata": {
+    "collapsed": false,
+    "pycharm": {
+     "name": "#%% md\n"
+    }
+   }
   },
   {
    "cell_type": "code",