Document extensibility of prior engine through framework

[tramsauer]

The extensibility of the prior enginge with the abstraction allows users to write their own code to be evaluated from the prior engine, like tonio stated:

From #19 (closed because duplicate and split up in multiple sub issues):
Tonio:

The framework, however, would go beyond that. It would allow users to not only add their own data, but to write their own code which can be interpreted by the prior engine.

This functionality must be documented.