Add explicit note regarding __package__ in top level modules

pep-0366.txt

@@ -72,6 +72,12 @@ package or subpackage, the boilerplate will need to be updated
 manually. It has the advantage that this change need only be made
 once per file, regardless of the number of relative imports.
 
+Note that setting ``__package__`` to the empty string explicitly is
+permitted, and has the effect of disabling all relative imports from
+that module (since the import machinery will consider it to be a
+top level module in that case). This means that tools like ``runpy``
+do not need to provide special case handling for top level modules
+when setting ``__package__``.
 
 Rationale for Change
 ====================