The company releases the information on its website and social media accounts. You may get a notification regarding that when it is available for all. When any new features are launched, everyone jumps into it to use it, this could be the other way where you will get Instagram  Notes from the friends who have listed your name in their ‘Close friends’ list. The possible way to know that the Instagram Notes feature is released is by checking any updates available in the Play store or Apple store.