getting-started: document members-only unencrypted Matrix space + admin-only invites

getting-started.md

@@ -73,6 +73,12 @@ privacy, and direction happen on our **Loomio forum**:
     rather use an existing account for co-op rooms, set it on your
     [dashboard](https://dashboard.inference.coop) under **Matrix chat** (one
     extra account) — invitations will go there instead. Both work either way.
+
+  **Members-only, readable history.** The space is invite-only and unencrypted
+  so every member can read the full history (including messages from before
+  they joined). Invites are restricted to admins — so while any member is
+  welcome to bring a collaborator or guest, an admin sends the invite rather
+  than members handing the space to outsiders directly.
 - See the [Charter](charter.md) for how governance is structured.
 - See [Open questions](open-questions.md) for the decisions currently on the table.