Document infra announcement channel membership syncing and pining etiquette

src/infra/docs/internal-announcements.md

@@ -8,6 +8,36 @@ signal-to-noise ratio_.
 By _internal facing_, we mean infrastructure that primarily is used by or only impacts project team
 members and other specific project-internal users (such as [Dev Desktop](./dev-desktop.md) users).
 
+## Syncing membership before posting an infra announcement
+
+Currently, since [`#t-infra/announcements`] is *not* managed through the `team` repo as it is a
+public Zulip stream, an infra team member looking to post an infra announcement should first
+manually resync the following Zulip groups to the stream:
+
+- `T-all` (all project team members)
+- `gsoc-contributors`, since they have access to Dev Desktops.
+- `ospp-contributors`, likewise.
+- `cloud-compute-users`, likewise.
+
+This can be done in Zulip through:
+
+> Channel Settings > t-infra/announcements > Subscribers > Add subscribers
+
+Note that Zulip admin privileges are needed to perform this action.
+
+## Announcement pinging etiquette
+
+Except for crucial announcements affecting _all_ project members, **avoid** general `T-all` Zulip
+group pings. `T-all` members will already be subscribed to the announcement channel unless they
+explicitly leave.
+
+Prefer either:
+
+- Just making an announcement topic, without explicitly pinging all project members, or
+- Only use *targeted* pings for specific project teams when only specific project teams are
+  affected.
+
+
 
 [`#t-infra/announcements`]:
     https://rust-lang.zulipchat.com/#narrow/channel/533458-t-infra.2Fannouncements