Update README.md before archiving

[mosuem]

See dart-lang/tools#1208

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• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:markdown	7.2.3	ready to publish	v7.2.3

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[coveralls]

Pull Request Test Coverage Report for Build 12239055461

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.114%

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Totals
Change from base Build 12217170719:	0.0%
Covered Lines:	1558
Relevant Lines:	1621

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💛 - Coveralls