Mention WEB_CONCURRENCY update in the upgrade doc (#2402)
Mention WEB_CONCURRENCY update in the upgrade doc (#2402)

https://github.com/puma/puma/issues/2393#issuecomment-702359283
eileencodes/puma,puma/puma,puma/puma,eileencodes/puma,puma/puma,puma/puma,eileencodes/puma,eileencodes/puma