Implement new Changelog format

[Vyrastas]

We have an existing changelog, but it's had only minimal updates:
https://github.com/openweave/openweave-core/blob/master/CHANGELOG

Since we don't have a formal release process with versioning and notes, we should reformat the changelog to be a bit more comprehensive, and use it for all substantial feature and dependency changes that should be highlighted for end users.

Taking inspiration from:
https://keepachangelog.com/en/1.0.0/
https://github.com/olivierlacan/keep-a-changelog/blob/master/CHANGELOG.md

I can get it started with some of the recent updates made to the project.

[Vyrastas]

Obsolete