add FAQ entry regarding StrPromise

Author: mschoettle

README.md

@@ -291,6 +291,14 @@ 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, you can either cast the `Promise` to `str` (causing the translation to be evaluated), or use the `StrPromise` or `StrOrPromise` types from `django-stubs-ext` in type hints. Which solution to choose depends depends on the particular case. See [working with lazy translation objects] in the Django documentation for more information.
+
+If this is reported on Django code, please report an issue or open a pull request to fix the type hints.
+
 ## Related projects
 
 - [`awesome-python-typing`](https://github.com/typeddjango/awesome-python-typing) - Awesome list of all typing-related things in Python.