The note can be found at The Retreat, on a coffee table in a treehouse on the southern end of the location.
I'm sick and tired of Jennie caving to Dagger and those other psycho Blood Eagles.
They've taken so much from us and it keeps getting worse. We're running out of ammo, food, clean water, chems... at this rate we'll all be dead within the year.
Hell, if this keeps up I might have to stop by Dagger's Den and sign up. Better than starving to death in this stupid tree.