Switch from manual Changelog text file to GitHub Releases

[jm3]

GitHub has a nice release-tagging process that makes it fast to create annotated releases using git tags without having to maintain a separate Changelog text file.

As an example, compare this:

to: https://github.com/jm3/openRTB-spec/blob/master/CHANGELOG

I recommend tagging the branch at RC points and creating releases for any major revs, it's a lot easier to follow and avoids summarizing all your changes again in a Changelog, because the releases point to actual git commits.

I'm happy to set up the tags, as I did here.