I also would just give them advance notice. I used to send out a newsletter, which I may soon go back to - so I would put that on the front page in big, bold font too.
Putting into the contract actually then contractually holds you to it. So I wouldn't do that. The one thing I do know is that you want to make sure they see the notice! I give it to them in at least 3 different ways - because, hey - does anyone read their email anymore? LOL
