Add viewport addon to the Addon Gallery

docs/src/pages/addons/addon-gallery/index.md

@@ -46,6 +46,10 @@ Redirects console output (logs, errors, warnings) into Action Logger Panel. `wit
 
 With this addon, you can switch between background colors and background images for your preview components. It is really helpful for styleguides.
 
+### [Viewport](https://github.com/storybooks/storybook/tree/master/addons/viewport)
+
+Viewport allows your stories to be displayed in different sizes and layouts in [Storybook](https://storybookjs.org).  This helps build responsive components inside of Storybook.
+
 ## Community Addons
 
 You need to install these addons directly from NPM in order to use them.