Update readme for 5.0.0

[chilian]

https://github.com/rspec/rspec-rails/releases/tag/v5.0.0
https://rubygems.org/gems/rspec-rails/versions/5.0.0

[pirj]

Please correct me if I'm totally wrong with my understanding of the versioning strategy:
rspec-rails 4.0 is out with Rails 6.0 support
rspec-rails 4.1 supports Rails 6.1
rspec-rails 5.0 is an early version with bleeding edge in-development Rails 7 support

With this in mind it's hard to call 5-0-mainteance a stable release and recommend it to everyone who visits the front repo page.

main tracks changes that will be merged to both the stable 4-1-maintenance and released as 4.1.x, and 5-0-maintenance and released as 5.0.x.
Rails 7-specific changes will go to main, will be merged to 5-0-maintenance, but not 4-1-maintenance.

@JonRowe Is this correct?

[JonRowe]

This should have been cherry picked from 5-0-maintenance my bad, we releaed 5.0.0 with Rails 6.1 support @pirj, it drops support for Rails versions so needed to be a major version. 4.1 contains some minor enhancements only.