add django 3.1 docs (#5142)

added django 3.1 official documentation to free-programming-books.md

free-programming-books.md

@@ -2462,6 +2462,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [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)
+* [Django Official Documentation](https://buildmedia.readthedocs.org/media/pdf/django/3.1.x/django.pdf) (PDF) (3.1)
 * [Django RESTful Web Services](https://www.packtpub.com/free-ebooks/django-restful-web-services) - Gaston C. Hillar (Packt account *required*)
 * [Django Web Framework (Python)](https://developer.mozilla.org/en-US/docs/Learn/Server-side/Django) - MDN contributors
 * [Djen of Django](http://agiliq.com/books/djenofdjango/)