Add CLI update instructions to README

[cdfuller]

Added instructions for updating the GitHub Copilot CLI.

[Copilot]

Pull Request Overview

Adds documentation for updating the GitHub Copilot CLI to help users keep their installation current with the latest version.

• Added a new "Updating the CLI" section with npm update command

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_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[williammartin]

We recommend that people update using npm i -g @github/copilot, and that's what we show on CLI exit:

Would welcome you to change this PR to just update the Installation heading to indicate that it's the same for upgrading.

[andyfeller]

Closing for now due to staleness but also because we're working to streamline distribution. We can revisit as needed.