Update README with notebook modifications and setup info

Added information about modifications made by Andy Barrett and updated usage of xarray.DataTree.

Author: andypbarrett

notebooks/SMAP/README.md

@@ -22,11 +22,13 @@ All notebooks are self contained, so there is no particular order to run them.
 * Minor text edits to provide additional information where necessary.
 * Moved the tutorials to the standard NSIDC tutorials template. 
 
+The notebooks were modified by Andy Barrett of NSIDC to take advantage of `xarray.DataTree` and modern tooling.  This involved a rewrite of the notebooks and using version 9 of SPL3SMP.
+
 ### Set up
 
 To run the notebook provided in this folder, please see the [NSIDC-Data-Tutorials repository readme](https://github.com/nsidc/NSIDC-Data-Tutorials#readme) for instructions on several ways (using Binder, Docker, or Conda) to do this.
 
 ### Key Learning Objectives
 
 1. Use the `earthaccess` library to search for and download SMAP data.
-2. Use the `xarray` library to read in the HDF5 files and plot the variables on a map using `xarray` plotting methods and `cartopy`.
+2. Use the `xarray` library to read in the HDF5 files and plot the variables on a map using `xarray` plotting methods and `cartopy`.