ADR for the new model

[SKernchen]

The new model is described here, but should also be described in adr. Especially, the process and the postprocess step.

[notactuallyfinn]

Here it is stated that neither the process nor the postprocess step should have plugins.
I find that a bit weird.
Regarding the process step:

• It should be extensible with strategies and I didn't come up with a better solution then what I implemented here. I.e. I don't know why we would make the process step not pluginizable.

Regarding the postprocess step:

• I have no idea why the plugins should be removed, maybe because deposit plugins should define their own postprocess plugins. But I don't why we would want that like I mentioned here (I also argue (there) for restructuring the steps either way).