Document the guidelines for the community wide slack announcements channel

> it might also help for us to define/document what kinds of announcements are considered to be "applicable enough" to the whole community to warrant a post in this channel compared to something else.
>
> Things I think could be a good use for this channel:
> - Election announcements
> - CHAOSSCon announcements (in addition to public social media/external comms)
> - Major news or announcements/blog posts from the board (new partnerships with other orgs, board membership changes, incident response)
> - Infrastructure announcements that impact the whole community (incident response or rare platform/tooling changes for core community functions)
>
> maybe "stuff that would be worth putting in a timeline of CHAOSS project history" is a good litmus test - AKA stuff that can be described as "the community" as a whole did something, rather than "this part of the community did something"

_Originally posted by @MoralCode in [#3](https://github.com/chaoss/infrastructure/issues/3#issuecomment-4736600322)_

Splitting this off into a new issue so we remember to document the guidelines

Start by locating the repository's existing community or Slack documentation and review how similar guidance is organized. Document when announcements belong in the community-wide channel, using the listed examples and the project-history litmus test; done means contributors can distinguish community-wide announcements from narrower updates.

Written by the indexing model from the issue text.