Adding new Showcase example (with source)

Author: Pedro Duarte

README.md

@@ -151,6 +151,7 @@ Websites built with Gatsby:
 * [Nexit](https://nexit.sk/)
 * [ERC dEX](https://ercdex.com)
 * [iContract Blog](https://blog.icontract.co.uk)
+* [Dona Rita](https://donarita.co.uk) ([source](https://github.com/peduarte/dona-rita-website))
 
 ## Docs