```ruby
# frozen_string_literal: true

require "paper_trail/version_concern"

module PaperTrail
  # This is the default ActiveRecord model provided by PaperTrail. Most simple
  # applications will only use this and its partner, `VersionAssociation`, but
  # it is possible to sub-class, extend, or even do without this model entirely.
  # See the readme for details.
  class Version < ::ActiveRecord::Base
    include PaperTrail::VersionConcern
  end
end
```

Clarify that VersionAssociation is in separate gem
user
```ruby
# frozen_string_literal: true

require "paper_trail/version_concern"

module PaperTrail
  # This is the default ActiveRecord model provided by PaperTrail. Most simple
  # applications will use this model as-is, but it is possible to sub-class,
  # extend, or even do without this model entirely. See documentation section
  # 6.a. Custom Version Classes.
  #
  # The paper_trail-association_tracking gem provides a related model,
  # `VersionAssociation`.
  class Version < ::ActiveRecord::Base
    include PaperTrail::VersionConcern
  end
end
```
assistant