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.
Sorry, one last thing: Could you add instructions to
> copy ``doc/release/release_template.txt`` to a new ``doc/release/release_dev.txt``