add text to docs

tools/generate_domain_files/docs/index.md

@@ -23,4 +23,14 @@ https://e3sm.org/resources/tools/other-tools/e3sm-unified-environment/
 Alternatively, a simple conda environment can be created with the following command:
 ```
 conda create --name example_env --channel conda-forge xarray numpy numba scikit-learn netcdf4
+```
+
+## Map File Generation
+
+The map file used to generate the domain files can be created a few different ways.
+For a typical E3SM configuration we recommend using a conservative, monotone map.
+Here is an example command that can be used to generate one (as of NCO version 5.2.2)
+
+```
+ncremap -5 -a traave --src_grd=${OCN_GRID} --dst_grd=${ATM_GRID} --map_file=${MAP_FILE}
 ```