If my boss posts an employment policy addendum on the back of a door, does it apply to the employees?

My boss posted an addendum to employee policy on the back of a door in a room that doubles as a break and storage room. I wanted to know if that means that the new policy now applies to employees just because it has been posted. Additionally, if they ask me to sign the addendum and I refuse, does the policy apply to me? The policy regards changing the how much notice employees must give before quitting from 2 weeks to 30 days. Also, we are hourly and not salary employees.