Split release notes into a CHANGELOG.md file

Split release notes into a CHANGELOG.md file

This patch splits out release notes into a separate dedicated file for
consistency with the puppetlabs puppet modules. Additionally, this
patch improves the release notes by:

- Fixing the tense to be consistent with commit message standards ("Add
  new feature" instead of "Added new feature" or "Adds new feature")
- Breaking up changes into categories to make it easier for operators
  to know what happened in the change: backwards-incompatible changes,
  features, bugfixes, and maintenance commits
- Linewrapping to 80 chars
- Adding release dates, formatted according to ISO-8601

Change-Id: Ic8be387f8b3d7dac0dd57b2b36b4a0d07696f55b