add FAQ entry regarding StrPromise

Author: mschoettle

README.md

@@ -291,6 +291,23 @@ def foo(obj: object) -> None:
         ...
 ```
 
+### Why am I getting incompatible argument type mentioning `_StrPromise`?
+
+The lazy translation functions of Django (such as `gettext_lazy`) return a `Promise` instead of `str`. These two types [cannot be used interchangeably](https://github.com/typeddjango/django-stubs/pull/1139#issuecomment-1232167698). The return type of these functions was therefore [changed](https://github.com/typeddjango/django-stubs/pull/689) to reflect that.
+
+If you encounter this error in your own code, use `StrPromise` or `StrOrPromise` from `django-stubs-ext`.
+
+```python
+from django.utils.translation import gettext_lazy as _
+
+from django_stubs_ext import StrOrPromise
+
+errors: list[StrOrPromise] = []
+errors.add(_('Error message'))
+```
+
+If this is reported on Django code, please report an issue or open a pull request.
+
 ## Related projects
 
 - [`awesome-python-typing`](https://github.com/typeddjango/awesome-python-typing) - Awesome list of all typing-related things in Python.