Add documentation for installing WP-CLI packages #284

Job	Run time
code-quality / PHPCS	18s
code-quality / PHPStan	20s
code-quality / Lint GitHub Actions workflows	6s
code-quality / Spell check	8s
code-quality / Lint Gherkin Feature files	16s
code-quality / Lint PHP files	17s
	1m 25s

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