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.
Could you add the boiler-plate introduction here as well? Here's what was in the last announcement:

> We're happy to announce the release of scikit-image v0.10.0!
>
>scikit-image is an image processing toolbox for SciPy that includes algorithms
for segmentation, geometric transformations, color space manipulation,
analysis, filtering, morphology, feature detection, and more.
>
>For more information, examples, and documentation, please visit our website:
>
>   http://scikit-image.org