add some more rubric

Author: Andy Barrett

notebooks/SMAP/smap_as_datatree.ipynb

@@ -20,7 +20,9 @@
    "metadata": {},
    "source": [
     "## 1. Overview  \n",
-    "In this tutorial, we will read the SMAP data we downloaded in 1.0 Download SMAP data notebook, read the data into an `xarray.DataTree`, create a map of soil moisture and plot a time-series soil moisture for a location on Earth."
+    "In this tutorial, we will read the SMAP data we downloaded in 1.0 Download SMAP data notebook, read the data into an `xarray.DataTree`, create a map of soil moisture and plot a time-series soil moisture for a location on Earth.\n",
+    "\n",
+    "`xarray.DataTree` was created to work with hierachical datasets.  Hierachical datasets are those datasets that use tree-like, nested, directory structures to group and store complex data.  The SMAP Level-3 is a relatively simple hierachical data structure, with only two groups; one group for data from AM satellite overpasses and one group for data from PM satellite overpasses.  To make working with this data easy, we also add meaningful names for the dimensions of the data and geospatial coordinates.  "
    ]
   },
   {