Clarify version to use with Rails 6.0 (#2498)
Clarify version to use with Rails 6.0 (#2498)

* Update README.md

By reading the README, it wasn't clear to me what version I'm supposed to use with Rails 6.0 and, to me "Rails from 5.0 to 6.0" means Rails 6.0 is included. After I read [versioning-strategy](https://github.com/rspec/rspec-rails/blob/main/rfcs/versioning-strategy.md), I understand rspec-rails 5.x have to used for Rails 6 and above. Also, the reasons of this rule wasn't clear.
rspec/rspec-rails,rspec/rspec-rails,rspec/rspec-rails