added a django ebook (#2507)

free-programming-books.md

@@ -1926,6 +1926,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 * [Effective Django](http://www.effectivedjango.com) (1.5)
 * [How to Tango With Django](http://www.tangowithdjango.com/book17/) (1.7)
 * [Test-Driven Web Development with Python](http://www.obeythetestinggoat.com/pages/book.html#toc) (1.8)
+* [The Django Book](https://djangobook.com/the-django-book/) (1.8 & 1.11)
 
 
 #### Flask