Changelog could use some motivation for the idea of variants and why they're used:

```
Introducing Variants

The request variant is a specialization of the request format, like :tablet, :phone, or :desktop.

We often want to render different html/json/xml templates for phones, tablets, and desktop browsers. Variants make it easy.

You can set the variant in a before_action:
    request.variant = :tablet if request.user_agent =~ /iPad/

Respond to variants in the action just like you respond to formats:
    respond_to do |format|
      format.html do |html|
        html.tablet # renders app/views/projects/show.html+tablet.erb
        html.phone { extra_setup; render ... }
      end
    end

Provide separate templates for each format and variant:
    app/views/projects/show.html.erb
    app/views/projects/show.html+tablet.erb
    app/views/projects/show.html+phone.erb
```