The quickest way to add this is to enable the “Custom Content Types” in the Jetpack plugin. Then go to “Settings → Writing” and under “Your Custom Content Types” check the “Testimonials” option. You will see a new custom post type called “Testimonials” in the WordPress Dashboard. Then, you can use the Custom Query Widget to display testimonials in a sidebar. From the theme customizer you can also configure a page to act as an archive for testimonials.