explain domain substructuring block

LDDsimulation/domainSubstructuring.py

-"""Class domainSubstructuring, Copyright 2020, David Seus
+"""Class domainSubstructuring and example substructurings, Copyright 2020, David Seus
 
 Provides the base class and example classes for domain substructurings used
 for LDD simulations within the LDDsimulation class.

Usecases/README.md

@@ -273,6 +273,15 @@ adjacent_subdomains = substructuring.adjacent_subdomains
 subdomain_def_points = substructuring.subdomain_def_points
 outer_boundary_def_points = substructuring.outer_boundary_def_points
 ~~~
+This block sets the domain of simulation and its subdomain substructuring.
+In this block you only need to change the line
+~~~
+substructuring = dss.twoSoilLayers()
+~~~
+This block calls constructors of substructurings. These are classes defined in
+`../LDDsimulation/domainSubstructuring.py`.
+If you want to define your own substructurings have a look in that file.
+Some explantion is given in that file on how to define substructuring classes. 
 
 ### MODEL CONFIGURATION
 ~~~python