Documenting how to create and test a command that creates Java code

The idea is to document commands that create new code (new commands). There are several level of complexity and we should go step by step :

1. A command to create a new constraint payload is quite simple. It only generates Java code to a specific directory
2. A command to create a new package info is a bit tricker because the generate Java code is not a class per-se
3. A command to create a new constraint validator have several parameters that can be used (type and targetConstraint). The case of the JPA Listener is also important because the command taks a choice of parameters (POST_UPDATE, PRE_PERSIST...)