Improvements to auto-generated CHANGELOG.md

[mmcky]

Suggested improvement to CHANGELOG.md file generator from @oyamad

This is an issue of the software, but it would be nice if the names of all the collaborators (committers) show up as contributors of a PR; see for example

https://github.com/QuantEcon/QuantEcon.py/pull/403/files#diff-4ac32a78649ca5bdd8e0ba38b7006a1eR9

where among the four committers, only the name of the person who opens the PR appears.