add note about why to filer with -f

README.md

@@ -59,4 +59,6 @@ config/chat_list.txt
 
 You might want to run the command once without filtering, then paste the chat
 name column from the spreadsheet into config/chat_list.txt and narrow down to
-only the ones you care about and run it again with -f.
+only the ones you care about and run it again with -f. This will ensure the
+output spreadsheet only has columns for users in the groups you care about, and
+not every single user you ever saw on Signal.