add default image for hero

exampleSite/config.toml

@@ -24,3 +24,4 @@ enableRobotsTXT = true
   youtube = ""
   # choose a background color from any on this page: http://tachyons.io/docs/themes/skins/ and preface it with "bg-"
   background_color_class = "bg-black"
+  featured_image = "/images/gohugo-default-sample-hero-image.jpg"