[Fix #5685] Add information on Rails version to README
[Fix #5685] Add information on Rails version to README

The Rails cops don't support Rails versions lower than 4.0. This
change adds a note about it in the `README`.

deivid-rodriguez/rubocop,tejasbubane/rubocop,bbatsov/rubocop,akihiro17/rubocop,jfelchner/rubocop,rrosenblum/rubocop,tdeo/rubocop,mikegee/rubocop,panthomakos/rubocop,tejasbubane/rubocop,sue445/rubocop,tdeo/rubocop,scottmatthewman/rubocop,vergenzt/rubocop,jmks/rubocop,maxjacobson/rubocop,meganemura/rubocop,smakagon/rubocop,bquorning/rubocop,mikegee/rubocop,rrosenblum/rubocop,palkan/rubocop,jfelchner/rubocop,tejasbubane/rubocop,panthomakos/rubocop,bbatsov/rubocop,maxjacobson/rubocop,bquorning/rubocop,tdeo/rubocop,vergenzt/rubocop,deivid-rodriguez/rubocop,maxjacobson/rubocop,meganemura/rubocop,petehamilton/rubocop,akihiro17/rubocop,sue445/rubocop,palkan/rubocop,smakagon/rubocop,palkan/rubocop,rrosenblum/rubocop,haziqhafizuddin/rubocop,haziqhafizuddin/rubocop,meganemura/rubocop,petehamilton/rubocop,panthomakos/rubocop,petehamilton/rubocop,sue445/rubocop,jmks/rubocop,mikegee/rubocop,vergenzt/rubocop,jmks/rubocop,scottmatthewman/rubocop,jfelchner/rubocop,akihiro17/rubocop,bquorning/rubocop,deivid-rodriguez/rubocop