Add docs about "type[Model] does not have objects attribute" (#2519)

Refs #1684

Author: sobolevn

README.md

@@ -310,6 +310,36 @@ reveal_type(settings.EXISTS_AT_RUNTIME)  # N: Any
 reveal_type(settings.MISSING)  # E: 'Settings' object has no attribute 'MISSING'
 ```
 
+### How to use `type[Model]` annotation with `.objects` attribute?
+
+Let's say you have a function similar to this one,
+which accepts a model type and accesses its `.object` attribute:
+
+```python
+from django.db import models
+
+def assert_zero_count(model_type: type[models.Model]) -> None:
+    assert model_type.objects.count() == 0
+```
+
+This code will raise an error from mypy:
+
+```
+error: "type[Model]" has no attribute "objects"  [attr-defined]
+```
+
+It is a common problem: some `type[models.Model]` types won't have `.objects` available.
+Notable example: [abstract models](https://docs.djangoproject.com/en/5.1/topics/db/models/#abstract-base-classes).
+See [the reasoning here](https://github.com/typeddjango/django-stubs/issues/1684).
+
+So, instead for the general case you should write:
+
+```python
+def assert_zero_count(model_type: type[models.Model]) -> None:
+    assert model_type._default_manager.count() == 0
+```
+
+
 ## Related projects
 
 - [`awesome-python-typing`](https://github.com/typeddjango/awesome-python-typing) - Awesome list of all typing-related things in Python.