You don't have to say much, it doesn't have to be a manifesto, and it doesn't have to exist to take away their pain. Tell them you love them, tell them how much they meant to you...those two things right there are worth more than you will ever know. If you have some explanation..that may help too.
But I can tell you, having lost the most important person in the world to me to suicide, having his note saying how much he loved me...I treasure more than any of my physical possessions.
Nothing about your passing will be easy for your family and friends, the people who love you, but give them comfort in whatever small way you can.
I promise you, your note, no matter how small, will mean a lot to them.
I keep my boyfriends note on my dresser, I read it often, even though I have thousands of messages and memories from him telling me how much he loved me...having that note say the same thing...I can't say why but it really means something to me.
Nothing you can say will take away their pain, you have to know that, but giving them something, even a few words, will mean a lot.