Add a warning before the compute-intensive cell in ch 03

03_classification.ipynb

@@ -1614,6 +1614,13 @@
     "knn_clf.predict([some_digit])"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**Warning**: the following cell may take a very long time (possibly hours depending on your hardware)."
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 72,