Add reference to the issue in the docstring.

importlib_resources/future/adapters.py

@@ -42,7 +42,7 @@ def wrapper(*args, **kwargs):
 
 def _skip_degenerate(reader):
     """
-    Mask any degenerate reader.
+    Mask any degenerate reader. Ref #298.
     """
     is_degenerate = (
         isinstance(reader, _adapters.CompatibilityFiles) and not reader._reader