Update docs/source/settings.rst

Co-authored-by: Przemek Lewandowski <haxoza@users.noreply.github.com>

docs/source/settings.rst

@@ -41,6 +41,15 @@ for this setting to work as expected.
 If you don't want to roll out your own authentication backend, ``LoginFieldBackend`` has been prepared.
 It works the same as ``ModelBackend``, but it is using ``LOGIN_FIELD`` instead of ``User.USERNAME_FIELD``.
 
+To make your code backward compatible with previous Djoser versions, add this auth backend to your Django settings:
+
+.. code-block:: python
+
+    AUTHENTICATION_BACKENDS = [
+        "djoser.auth_backends.LoginFieldBackend",
+    ]
+
+Please notice that it is not recommended way of authentication and it may be removed in the future Djoser versions.
 **Default**: ``User.USERNAME_FIELD`` where ``User`` is the model set with Django's setting AUTH_USER_MODEL.
 
 .. warning::