Document revised announcement process

[ct-martin]

The RSS feed for announcements gets watched by an IFTTT hook that posts to the Google Group, however, CampusGroups is not automated and doesn't play well with others. We might get that working sometime in the future, but it won't be the near future. The new process for posting announcements will be something like:

1. Announcement drafted in PR. This allows convenient reviewing.
2. Announcement gets approved by two people on eboard.
3. When the announcement is merged, the merger will also be responsible for posting to CampusGroups [& Slack]

[Tjzabel]

@zethra you guys are now in charge of how you want to do weekly general meeting announcements. I would recommend adding a small blurb to the runbook for how meeting announcements are to be sent out.