docs: add update instructions for npm and Homebrew

[umyunsang]

Summary

• Add update commands (npm, Homebrew) to README Installation section

Test plan

• CI passes

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 753f4aaf-93bf-4c5c-976b-7cd54866d680

📥 Commits

Reviewing files that changed from the base of the PR and between a72c419 and 63cd746.

📒 Files selected for processing (1)

• README.md

Warning

.coderabbit.yaml has a parsing error

The CodeRabbit configuration file in this repository has a parsing error and default settings were used instead. Please fix the error(s) in the configuration file. You can initialize chat with CodeRabbit to get help with the configuration file.

💥 Parsing errors (1)

Validation error: String must contain at most 250 character(s) at "tone_instructions"

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/add-update-instructions

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