Add description to features_count and labels_count

Author: Brok Bucholtz

intro-to-tensorFlow/intro_to_tensorflow.ipynb

@@ -347,7 +347,9 @@
    },
    "outputs": [],
    "source": [
+    "# All the pixels in the image (28 * 28 = 784)\n",
     "features_count = 784\n",
+    "# All the labels\n",
     "labels_count = 10\n",
     "\n",
     "# TODO: Set the features and labels tensors\n",