Add CHANGELOG

[tajmone]

With the upcoming v1.1.0 update we need to start tracking changes between releases of the AsciiDoc port of Hugo Book.

We could either add a "Changes" section in the main README or create an independent CHANGELOG.md document; the former solution seems better to start off with, and when the list becomes long we could then move it to the independent CHANGELOG.md document.

• Create a CHANGELOG label to track issues relating to updating the ChangeLog.
• Add to main README.md a "Changes" section:
  • Document v1.0.0 (Initial release).
  • Document all significant changes for upcoming v1.1.0 — see Update Changelog for v1.1.0 #69 for this.

As for the details of the changes list, we could the list brief by providing reference links to the various Issues and commits, and just keep a brief resume in the text description.