Add explanation / instructions for required 2nd Python-ast.c submit.

pep-0306.txt

@@ -71,6 +71,13 @@ Checklist
 
     __ Documentation must be written!
 
+    __ After everything's been checked in, you're likely to see a new
+       change to Python/Python-ast.c.  This is because this
+       (generated) file contains the SVN version of the source from
+       which it was generated.  There's no way to avoid this; you just
+       have to submit this file separately.
+
+
 References
 
     [1] SF Bug #676521, parser module validation failure