It is same as before, now only change is you can have cache dir inside q2a dir. well, if you want you can still have this dir outside www/html dir.
1. add path in qa-config.php file , like /var/www/html/urq2a/qa-cache/
2. go to admin panel and enable cache.
thats all. I enabled in a same way on site https://meta.question2answer.info
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Jan 12, 2019: Q2A 1.8.3 released!