fixup! In the upgrade guide mention min Ruby and Rails versions

Author: pirj

features/upgrade/README.md

@@ -1,5 +1,9 @@
 # Upgrading from rspec-rails 3.x to version 4
 
-There is nothing that needs to be done to upgrade from RSpec Rails 3 to version 4.0.
+RSpec Rails 4 requires Ruby 2.2.
+
+Please also keep in mind that even though RSpec Rails 4 has soft support for Rails 4.2, it's recommended to use Rails 5+.
+
+Other than that, there is nothing that needs to be done to upgrade from RSpec Rails 3 to version 4.0.
 
 If you are upgrading from an earlier version of RSpec Rails, please consult [the upgrading 2.x to 3.x guide](https://relishapp.com/rspec/rspec-rails/v/3-9/docs/upgrade).