add CHANGES file

[raffenet]

Pull Request Description

Highlight additions and changes since the 0.1x branch was made.

Expected Impact

Author Checklist

• Reference appropriate issues (with "Fixes" or "See" as appropriate)
• Commits are self-contained and do not do two things at once
• Commit message is of the form: module: short description and follows good practice
• Add comments such that someone without knowledge of the code could understand
• Have read and agree to the Yaksa CLA terms (https://github.com/pmodels/yaksa/wiki/Yaksa-Contributor-License-Agreement)

[raffenet]

I tried to capture as many significant changes as I could from the beginning of 0.1x to 0.2x.

git shortlog --no-merges 536645c33e21fb2770d8071c0e9b329095915e5a..0.2x

[hzhou]

LGTM!