add hack language (#2580)

* add hack language

closes #2097

* Whoops! Wrong list.

* aaaaarg

free-programming-books.md

@@ -54,6 +54,7 @@
   * [Gradle](#gradle)
   * [Grails](#grails)
   * [Spock Framework](#spock-framework)
+* [Hack](#hack)
 * [Hadoop](#hadoop)
 * [Haskell](#haskell)
 * [Haxe](#haxe)
@@ -1187,6 +1188,11 @@
 * [Spock Intro](http://edgibbs.com/spock-intro-a-bdd-testing-framework-in-groovy/) - Ed Gibbs
 
 
+### Hack
+
+* [Hack Documentation](https://docs.hhvm.com/hack/)
+
+
 ### Hadoop
 
 * [Cloudera Impala](http://www.cloudera.com/content/www/en-us/resources/aboutcloudera/cloudera-impala-ebook.html) - John Russel