add updated django (v 2.2.x) link to pdf docs (#3246)

there were only links to v1.x django docs, so I added a link to the django 2.2 pdf docs

url = https://buildmedia.readthedocs.org/media/pdf/django/2.2.x/django.pdf

free-programming-books.md

@@ -2189,6 +2189,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Django Official Documentation](https://media.readthedocs.org/pdf/django/1.7.x/django.pdf) (PDF) (1.7)
 * [Django Official Documentation](https://media.readthedocs.org/pdf/django/1.9.x/django.pdf) (PDF) (1.9)
 * [Django Official Documentation](https://media.readthedocs.org/pdf/django/1.10.x/django.pdf) (PDF) (1.10)
+* [Django Official Documentation](https://buildmedia.readthedocs.org/media/pdf/django/2.2.x/django.pdf) (PDF) (2.2)
 * [Djen of Django](http://agiliq.com/books/djenofdjango/)
 * [Effective Django](http://www.effectivedjango.com) (1.5)
 * [How to Tango With Django](http://www.tangowithdjango.com/book17/) (1.7)