Very simple. The moderator of your sub should create a new post and set it as an announcement by selecting "Post topic as-Announcement".
Setting it as an announcement will ensure that the system never removes it automatically.
Give the topic a title such as "Monthly Newsletter".
Then each month someone would post a reply to that topic with the contents of the newsletter.
As a bonus, subdivision residents can select the "Watch this topic" option and will get an email notification automatically when a new newsletter is posted.