add graphql tutorial (EbookFoundation#2887)

Author: Alexey Gorelov

free-programming-interactive-tutorials-en.md

@@ -137,6 +137,7 @@
 * [CodeCombat](http://codecombat.com) - Python, JavaScript, CoffeeScript, Clojure, Lua, Io
 * [Codility](https://codility.com/programmers/)
 * [Python Tutor](http://pythontutor.com) - Python, Java, JavaScript, TypeScript, Ruby, C, C++
+* [The Fullstack Tutorial for GraphQL](https://www.howtographql.com)
 
 
 ### Lisp