Add plainmarkdown breaking change entry to v0.19.0 changelog

[SBGoods]

Closes: #355

PR #332 refactored the mdplain package and reimplemented the plainmarkdown function using the yuin/goldmark library. The purpose of the plainmarkdown function is to strip out markdown elements to render the input as plain text, primarily for the YAML frontmatter. The previous implementation of plainmarkdown still kept some markdown formatting elements such as links and bulletpoints but removed others like headers. The new implementation in PR #332 now strips out all markdown formatting elements but a changelog entry associated with this change was not included in the v0.19.0 release.

This PR retroactively adds a changelog entry into the Changie 0.19.0.md changelog file and CHANGELOG.md

[bflad]

Looks good to me 🚀

[github-actions]

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