[skip ci] add doc about modules

doc/Modularization.md

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+# Modules 
+
+## Modules and their possible relations
+
+![modularization.png](resources/modularization.png)
+
+
+#### Modules
+
+A,B,C: Modules with both domain and infrastructure layer code - common type of modules   
+D: Module with only domain: can sometimes make sense if only domain logic and no infrastructure logic is required   
+E: Module with only infrastructure: usually utility modules that just provide a collection of infrastructure functionality
+
+#### Interactions
+
+1. Domain calls (a function in) the infrastructure of the same module - common practice within a module
+1. Domain calls (a function in) the domain another module - common practice between modules
+1. Infrastructure calls infrastructure of another module - usually not recommended
+1. Domain calls infrastructure in another module - can make sense in some cases e.g. if module D just needs some low-level function of module D. However where possible calling domain of module C should be preferred
+1. Domain calls infrastructure in another module, which only has infrastructure - common practice for calling utility modules, which don't have a domain.