Add "How to GraphQL" to list of showcases (#2905)

Author: nikolasburk

README.md

@@ -71,6 +71,7 @@ Websites built with Gatsby:
 * [Mannequin.io](https://mannequin.io) ([source](https://github.com/LastCallMedia/Mannequin/tree/master/site))
 * [API Platform](https://api-platform.com) ([source](https://github.com/api-platform/website))
 * [Bottender Docs](https://bottender.js.org/) ([source](https://github.com/bottenderjs/bottenderjs.github.io))
+* [How to GraphQL](https://www.howtographql.com/) ([source](https://github.com/howtographql/howtographql))
 * [greglobinski.com](https://greglobinski.com) ([source](https://github.com/greglobinski/greglobinski-com))
 
 ## Docs