Add guideline for undocumented objects. (#1938)

Also add Ivan as a typeshed core dev (since he is in the group).

CONTRIBUTING.md

@@ -82,6 +82,7 @@ At present the core developers are (alphabetically):
 * David Fisher (@ddfisher)
 * Łukasz Langa (@ambv)
 * Jukka Lehtosalo (@JukkaL)
+* Ivan Levkivskyi (@ilevkivskyi)
 * Matthias Kramm (@matthiaskramm)
 * Greg Price (@gnprice)
 * Guido van Rossum (@gvanrossum)
@@ -133,6 +134,15 @@ included in ``__all__`` (if present), and whose names do not start with an
 underscore are more likely to merit inclusion in a stub. If in doubt, err
 on the side of including more objects.
 
+**NEW:** Sometimes it makes sense to include non-public objects
+in a stub.  Mark these with a comment of the form ``# undocumented``.
+See the [motivation](https://github.com/python/typeshed/issues/1902).
+Example:
+
+```python
+def list2cmdline(seq: Sequence[str]) -> str: ...  # undocumented
+```
+
 ### Using stubgen
 
 Mypy includes a tool called [stubgen](https://github.com/python/mypy/blob/master/mypy/stubgen.py)