clarify relevance of datalist generator notebook

Author: DanielNobbe

auto3dseg/notebooks/msd_datalist_generator.ipynb renamed to auto3dseg/notebooks/msd_crossval_datalist_generator.ipynb

@@ -22,6 +22,13 @@
     "# Datalist Generator"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "This notebook contains an example to add cross-validation folds to an existing Medical Segmentation Decathlon datalist, in this case the one of Task09_Spleen."
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {