Store: Make mention of declined.txt in comments.

src/calibre/gui2/store/__init__.py

@@ -43,6 +43,8 @@ class StorePlugin(object): # {{{
     The easiest way to handle affiliate money payouts is to randomly select
     between the author's affiliate id and calibre's affiliate id so that
     70% of the time the author's id is used.
+    
+    See declined.txt for a list of stores that do not want to be included.
     '''
 
     def __init__(self, gui, name):