Add Type annotations developer guideline

docs/development/developer-guide.rst

@@ -131,3 +131,22 @@ simple Python script ``tools/vendor.py``.
 To refresh the dependencies, run the following command::
 
     $ tox -e vendor
+
+----------------
+Type annotations
+----------------
+
+Setuptools makes use of inferred return annotations to reduce verbosity,
+especially for complex return types. Explicit return types can be added for
+functions whose inferred return type contains ``Any``, or that are not checked by
+mypy (ie.: contains no parameter *and* no return type, see
+[python/mypy#4409](https://github.com/python/mypy/issues/4409),
+[python/mypy#10149](https://github.com/python/mypy/issues/10149) and
+[python/mypy#6646](https://github.com/python/mypy/issues/6646) ).
+
+Instead of typing an explicit return type annotation as
+``Generator[..., None, None]``, we'll prefer using an ``Iterator`` as it is more
+concise and conceptually easier to deal with. Returning a ``Generator`` with no
+``yield`` type or ``send`` type can sometimes be considered as exposing
+implementation details. See
+[Y058](https://github.com/PyCQA/flake8-pyi/blob/main/ERRORCODES.md#Y058).