also read/import product discounts

doc/development_documentation.md

@@ -57,12 +57,26 @@ to validate the XML part of the invoices.
 
 ![Architecture of the validator](ZUV-Architektur.svg "Graph of the architecture of the validator component")
 
-## New build
+## Aspects
+Apart from the fact that apart from 
+* the code
+* we need tests and apart from implementing it in 
 
-Target platform is java 1.17
+* the interface
+* usually we need functionality in or via the invoice class. 
+
+Reading should work for both
+* CII and
+* UBL
+
+And when writing, 
+* it should be readable as well, usually in the invoiceimporter, 
+* and it should be readable and writeable via Jackson (i.e. JSON)
 
 ## Build
 
+Target platform is java 1.17
+
 The package can be build with
 ```
 mvnw clean package