That's an easy one: you don't write a note at all. If there's nobody you care about enough to explain things to them and say goodbye, why bother?
I'm in the same boat. I've gone dark since January and nobody's come knocking so there's that. I probably won't even accounce it here: when it's time to go, I'll know, and I won't be sticking around any longer than I have to.
Sincerely,
CumbriaCTB