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.
Would it make sense to have a `doc/release/release_template.txt` that doesn't change and copy that as `doc/release/release_dev.txt` after bumping versions?