Another relatively popular CMS, Joomla is free and open-sourced. Used for websites across different industries and in varying sizes.
Joomla actually provides a default robots.txt on their website, see below. Depending on your site, however, you’ll want to perform a crawl to see if there’s anything else you’d like to add.
User-agent: *
Disallow: /administrator/
Disallow: /bin/
Disallow: /cache/
Disallow: /cli/
Disallow: /components/
Disallow: /includes/
Disallow: /installation/
Disallow: /language/
Disallow: /layouts/
Disallow: /libraries/
Disallow: /logs/
Disallow: /modules/
Disallow: /plugins/
Disallow: /tmp/