Issue #3621677: Add Queue UI usage location

Ishfaq Ahmadrequested to merge
issue/queue_ui-3621677:3621677-readme-should-say into 4.0.x

Adds a concise Usage section to README.md with the Queue manager administration location and route. Verified against queue_ui.links.menu.yml and queue_ui.routing.yml; git diff --check passes. Drupal.org issue: https://www.drupal.org/project/queue_ui/issues/3621677

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