Add note on avoiding DoubleRenderError in Rails to Readme

When a `Pundit::NotAuthorizedError` is raised in an `after_action` (Rails 4)/ `after_filter` (Rails 3), the response has already been rendered.

To avoid a `DoubleRenderError` being raised, the response body can be cleared before redirecting or rendering a new few. 

Tested only in the browser in Rails 4.0. Fixes #52.

Please add a comment if you know another solution so we can work it in.