add ventura-digital.de to sites made with gatsby

Author: Fetten

README.md

@@ -154,6 +154,7 @@ Websites built with Gatsby:
 * [Dona Rita](https://donarita.co.uk) ([source](https://github.com/peduarte/dona-rita-website))
 * [Andy Slezak](https://www.aslezak.com) ([source](https://github.com/amslezak))
 * [Odayaka](http://odayaka.net) ([source](https://github.com/odayakanaasa/odayaka))
+* [Ventura Digitalagentur Köln](https://www.ventura-digital.de)
 
 ## Docs