docs: add Homebrew as an installation option in README

[emaarco]

Summary

The README lists four installation methods (npm, pre-built binaries, Cargo, Nix) but omits Homebrew, which is the most common package manager on macOS. A formula already exists in homebrew-core:
https://github.com/Homebrew/homebrew-core/blob/main/Formula/g/googleworkspace-cli.rb

Suggested fix

Add the following to the installation section of the README:

brew install googleworkspace-cli