Clarify Rails 4.1+ support

Author: chancancode

README

@@ -15,3 +15,15 @@ prototype-rails supports RJS debugging. RJS responses are wrapped to catch
 exceptions, alert() them, and re-raise the exception. Debugging is disabled by
 default. To enable in development, set `config.action_view.debug_rjs = true`
 in config/environments/development.rb.
+
+---
+
+## Support for Rails 4.1 and above
+
+Unfortunately, due to limited manpower and resources, the Rails core team has
+not been able to confirm if this gem currently works with Rails 4.1 and above.
+If you have found any problems while upgrading your application, please report
+them at the [issue tracker](https://github.com/rails/prototype-rails/issues),
+or better yet, submit patches by sending a [pull request](https://github.com/rails/prototype-rails/pulls).
+
+In any case, this gem will *NOT* be officially supported on Rails 5.0 and above.