Update example notebook with a widget example.

examples/lab/example.ipynb

@@ -41,6 +41,18 @@
     "print('some more stdout text', flush=True)"
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "from ipywidgets import IntSlider\n",
+    "IntSlider() # slider appears at the top of the notebook"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},