Update README adapt to Rails 6 [skip ci]

[soartec-lab]

First of all, thank you for developing a great gem

Summary

In the change from rails5 to rails6, There were no changes to the ActiveRecord::Base.clear_all_connections! and ActiveRecord::Base.clear_active_connections! ActiveRecord methods called in this gem.

See: https://github.com/rails/rails/blame/e00f9c8aaea3fbedc2d19926da4f4575c12f0aaa/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

Therefore, I added to README that this gem works on rails6.

could you verification?

[kamipo]

FYI the section for Rails 5 was added in #6 since at that time Rails 5.0.0 have the race condition https://github.com/sonots/activerecord-refresh_connection/issues/5#issuecomment-231568014.

That was already fixed by https://github.com/rails/rails/pull/27057 and backported https://github.com/rails/rails/commit/fd21a5736b857a0f38c2617bf6e83515aeee755a in Rails 5.0.1.