Add a `CHANGELOG.md` file generated from github releases

[meliache]

Try to adhere loosely to the format from Keep a Changelog for the newer releases.

To generate the CHANGELOG.md from github releases, I used github-release-notes (gren) with the following custom template in my .grenrc.js (see gren options docs):

module.exports = {
    "template": {
        "release": function (placeholders) {
            // change date format to YY-mm-dd
            [day, month, year] = placeholders.date.split("/")
            iso_date = `${year}-${month}-${day}`
            // remove leading "v" from release
            release = placeholders.release.replace("v", "")
            return `## [${release}] - ${iso_date}\n${placeholders.body}`
        }
    }
}

This still looked bad because some github release notes had markdown headings which were h2 (##) or lower, hower ## is used by gren for release headings. So I hand-edited the CHANGELOG.md to adapt some heading levels and in the process also changed the newer releases to adhere more closely to the keepachangelog format.

Closes #145

[codecov-commenter]

Codecov Report

Merging #146 (4824fc4) into main (539f653) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #146   +/-   ##
=======================================
  Coverage   57.38%   57.38%           
=======================================
  Files          23       23           
  Lines        1516     1516           
=======================================
  Hits          870      870           
  Misses        646      646           

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