Update README.md (#372)

Added in my site built with Gatsby, thank you so much!

Author: jdhorwitz

README.md

@@ -65,6 +65,7 @@ it'll be converted to `/docs/index.html`.
 * [twnsnd.co](http://twnsnd.co/)
 * [Beau Han Xu](http://www.beauhanxu.com/)
 * [HalfAtheist](https://halfatheist.com/) ([source](https://github.com/halfatheist/halfatheist.github.io))
+* [joshuahorwitz.net](http://joshuahorwitz.net)
 * [Edit this file to add yours!](https://github.com/gatsbyjs/gatsby/blob/master/README.md)
 
 *Note, for the sites that have made their source available, you can