update the readme to clarify the package's goals

[devoncarew]

• update the readme to clarify the package's goals (what's in scope and what's out of scope)
• fix Update the readme to clarify the goals and non-goals of this package dart-lang/core#799
• prep for a publish

---

• 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:lints	2.1.1	ready to publish	v2.1.1

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

[devoncarew]

thanks for the feedback - updated