Changelog: Release notes should be added to persistant changelog

[benjamincharity]

Currently changes are defined in the release notes. Going forward, we should be appending these release notes to a document named CHANGELOG.md so that all release notes are easily searchable.

• [ ] generate or update CHANGELOG.md
• [ ] release notes should still be included in individual releases
• [ ] individual release notes should link to primary README.md

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• https://github.com/wmfs/pg-telepods/blob/c90761dec87203b4f9484d5cbcfa46440116b826/.releaserc.json#L17-L27
• https://github.com/npmkit/npmkit/blob/24f4e179672805412e2d43b389d5f48e408fb550/.releaserc.json#L10-L28

[benjamincharity]

Closed by #1400