This is the hardest part for me to decide on. On one hand it could be helpful to explain why I've done it to give people closure; on the other hand I worry that it will cause people more pain if they obsess over it and read too much into it.
For example, if I say, "please look after each other", my family might think I'm implying they didn't do a good enough job of looking after me.
I think I'll just leave something simple like "I'm truly sorry and I love you all very much."
At the end of the day, they're going to be faced with the fact that their much-loved daughter hated herself and her life, and died alone, out of her mind on drugs, hanging from a tree with a broken neck, at 19 years old. Nothing in the world can make that hurt less.