add icell for gluoncv/mxnet pip installation

Author: LauLauThom

tutorials/Tutorial3-SpeedingUp.ipynb

@@ -426,6 +426,16 @@
     "For this tutorial I'm using version 0.5.0."
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# Alternatively you can directly install the gluoncv and mxnet packages to your environment by running this cell\n",
+    "!pip install gluoncv mxnet"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 47,