Add release date to release notes

[rca]

It would be great if the release notes contained the release date. For example, v0.17.4, I had to drop to git log to get the date:

http://docs.saltstack.com/topics/releases/0.17.4.html

Thanks!

[cachedout]

The new (upcoming) release scheme will be date-based so it will be simple to tell when a release was cut. However, we really should back-date a couple of the release notes. Good idea.