New release note process

To make sure we have (1) have a nice list of release note changes, (2) we do not forget to add contributions in the release notes, I suggest to incrementally build the release notes with each PR. E.g., similar to the dev process in NumPy or scikit-learn.
Well, 0.11.1 development should happen on the 0.11.x branch, while 0.12.0 should happen on master, so it should be clear at that point which one is next. I think?