Document release requirements

[cornelius]

For incubation stage 2 we have defined that we require signed releases. We should make clear what that means and how to do it. There might be more requirements we want to apply to releases. This could be a section in the project handbook to make it clear how releases are recommended to be done. The incubation stage criteria could link to that. Maybe they also could be formulated clearer once we have a view on how releases should look like in detail. We still want to provide projects freedom to do it in the best way for them, so this shoudl be examples and general quality criteria, not mandating a specific procedure.

[cornelius]

Superseded by #144