completion: Add recommendation about where to put alias customization.

There was a discussion on the mailing list about upgrades and where to put it.

todo_completion

@@ -85,6 +85,9 @@ complete -F _todo todo.sh
 # If you define an alias (e.g. "t") to todo.sh, you need to explicitly enable
 # completion for it, too:
 #complete -F _todo t
+# It is recommended to put this line next to your alias definition in your
+# ~/.bashrc (or wherever else you're defining your alias). If you simply
+# uncomment it here, you will need to redo this on every todo.txt update!
 
 # If you have renamed the todo.sh executable, or if it is not accessible through
 # PATH, you need to add and use a wrapper completion function, like this: