add a note about constant conventions

pep-0008.txt

@@ -563,6 +563,12 @@ Naming Conventions
 
       Note: there is some controversy about the use of __names (see below).
 
+    Constants
+
+       Constants are usually declared on a module level and written in all
+       capital letter with underscores separating words.  Examples include
+       MAX_OVERFLOW and TOTAL.
+
     Designing for inheritance
 
       Always decide whether a class's methods and instance variables