Update supported Django versions in README.rst (jazzband#403)

README.rst

@@ -38,7 +38,7 @@ user. Although the package is optional, it improves account security control
 over ``django.contrib.sessions``.
 
 Compatible with modern Django versions. At the moment of writing that's
-including 2.2, and 3.0 on Python 3.5, 3.6, 3.7 and 3.8.
+including 2.2, 3.0, and 3.1 on Python 3.5, 3.6, 3.7 and 3.8.
 Documentation is available at `readthedocs.org`_.