add kernel information for gps in GPy notebook

gaussian_processes/GPy.ipynb

@@ -10,7 +10,9 @@
     "import numpy as np\n",
     "from matplotlib import pyplot as plt\n",
     "from scipy import stats\n",
-    "from scipy.stats import multivariate_normal"
+    "from scipy.stats import multivariate_normal\n",
+    "\n",
+    "# See kernels: https://www.cs.toronto.edu/~duvenaud/cookbook/"
    ]
   },
   {