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.

Author: GCorbel

README.md

@@ -8,9 +8,11 @@ They’re also specifications (or _specs,_ for short):
 detailed explanations of how the application is supposed to behave,
 expressed in plain English.
 
-Use **[`rspec-rails` 4.x][]** for Rails from 5.0 to 6.0.
-Use **[`rspec-rails` 3.x][]** for Rails earlier than 5.0.
-Use **[`rspec-rails` 1.x][]** for Rails 2.x.
+According to [RSpec Rails new versioning strategy][] use:
+* **[`rspec-rails` 5.x][]** for Rails 6.x.
+* **[`rspec-rails` 4.x][]** for Rails from 5.x or 6.x.
+* **[`rspec-rails` 3.x][]** for Rails earlier than 5.0.
+* **[`rspec-rails` 1.x][]** for Rails 2.x.
 
 [Build Status]: https://secure.travis-ci.org/rspec/rspec-rails.svg?branch=main
 [travis-ci]: https://travis-ci.org/rspec/rspec-rails
@@ -23,6 +25,8 @@ Use **[`rspec-rails` 1.x][]** for Rails 2.x.
 [`rspec-rails` 1.x]: https://github.com/dchelimsky/rspec-rails
 [`rspec-rails` 3.x]: https://github.com/rspec/rspec-rails/tree/3-9-maintenance
 [`rspec-rails` 4.x]: https://github.com/rspec/rspec-rails/tree/4-1-maintenance
+[`rspec-rails` 5.x]: https://github.com/rspec/rspec-rails/tree/5-0-maintenance
+[RSpec Rails new versioning strategy]: https://github.com/rspec/rspec-rails/blob/main/rfcs/versioning-strategy.md
 
 ## Installation