Added previous_step to README

I'm not sure if previous_step is part of your public api so figured this was the best way to find out and let others know.

I need this for my current app so that if a user makes changes while navigating backwards through the wizard the changes will be saved. Given two submit buttons "Previous" and "Next" it looks like this:

```
    @user.assign_attributes(user_params)
    if params[:commit] == "Next"
      @user.step = step
    else
      jump_to previous_step
    end
    render_wizard @sponsorship_application
```