Add warning about TAXIICollection bug

Related: #125.

docs/guide/taxii.ipynb

@@ -261,25 +261,65 @@
     "\n",
     "tc_store.add(ind)\n"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### Bug and Workaround\n",
+    "\n",
+    "You may get an error similar to the following when adding STIX objects to a TAXIICollectionStore or TAXIICollectionSink:\n",
+    "\n",
+    "```\n",
+    "TypeError: Object of type ThreatActor is not JSON serializable\n",
+    "```\n",
+    "\n",
+    "This is a known bug and we are working to fix it. For more information, see [this GitHub issue](https://github.com/oasis-open/cti-python-stix2/issues/125) In the meantime, try this workaround:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "tc_sink.add(json.loads(Bundle(ta).serialize()))"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Or bypass the TAXIICollection altogether and interact with the collection itself:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "collection.add_objects(json.loads(Bundle(ta).serialize()))"
+   ]
   }
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 2",
+   "display_name": "Python 3",
    "language": "python",
-   "name": "python2"
+   "name": "python3"
   },
   "language_info": {
    "codemirror_mode": {
     "name": "ipython",
-    "version": 2
+    "version": 3
    },
    "file_extension": ".py",
    "mimetype": "text/x-python",
    "name": "python",
    "nbconvert_exporter": "python",
-   "pygments_lexer": "ipython2",
-   "version": "2.7.12"
+   "pygments_lexer": "ipython3",
+   "version": "3.6.3"
   }
  },
  "nbformat": 4,