docs: add update instructions

[jordansilly77-stack]

Summary

• add an Updating section to README
• document Homebrew, npm, Cargo, and manual binary update paths
• include the required changeset

Testing

• git diff --check

Fixes #848

[changeset-bot]

🦋 Changeset detected

Latest commit: 0e6a8ee

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@googleworkspace/cli	Patch

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[github-actions]

This PR has been inactive for 72 hours. Closing to keep the queue clean.

[github-actions]

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