Update and correct mustachio README

[srawlins]

In preparing to add documention for an area of mustachio, I re-read these sections and made corrections and improvements.

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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.

[srawlins]

Ready for review, @dart-lang/analyzer-team but zero rush 😀

[pq]

Really nice!

[srawlins]

Thanks!