added Mastering Django book (#1993)

free-programming-books.md

@@ -1805,6 +1805,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 * [Django Official Documentation](https://media.readthedocs.org/pdf/django/1.9.x/django.pdf) (PDF) (1.9)
 * [Djen of Django](http://agiliq.com/books/djenofdjango/)
 * [Effective Django](http://www.effectivedjango.com) (1.5)
+* [Mastering Django](http://masteringdjango.com) (1.8)
 * [Tango With Django](http://www.tangowithdjango.com/book17/) (1.7)
 * [Test-Driven Web Development with Python](http://chimera.labs.oreilly.com/books/1234000000754/index.html) (1.8)