The note can be found inside the Burrows, on a table near a sleeping bag east of the market, past the chained red door.
Marcus - that son of a bitch. How could he? Kicking us out of our home... why? He's giving all of us a death sentence - the people he called family. After his 'speech', a few of the folks said "Why don't we fight? Why don't we do anything at all?" It would be foolish - Marcus' group is armed and prepared! They're all strong men, younger than most of us. We wouldn't stand a chance. As much as I hate it, we have to leave.
But we're not leaving empty-handed. I've talked it over with some of the elders. Tonight, we're going to take back some of the supplies the Burrow Boys 'took as tribute' and leave through the old tunnels. It's dangerous, but Marcus knows that and won't follow us. We can get to the pump station, seal the doors... maybe hack the security system. We can figure it out later. All I know is that if we stay here, we're dead.