Add public_folder to docs

docs/utilities.md

@@ -9,3 +9,11 @@ Just like other things in `kemal`, browser redirection is super simple as well.
   end
 ```
 _Make sure to receive `env` as param in defined route's block or you might end-up having compile-time errors._
+
+# Custom Public Folder
+
+Kemal mounts `./public` root path of the project as the default public asset folder. You can change this by using `public_folder`.
+
+```ruby
+  public_folder "path/to/your/folder"
+```