docs: add #design room to Matrix section

getting-started.md

@@ -91,6 +91,9 @@ privacy, and direction happen on our **Loomio forum**:
   (never any member-identifying details) and only the bot and admins can
   write — it's a read-and-follow channel. Questions go to the main space
   room.
+
+- **#design** — an open member room for co-op design discussions: website,
+  docs, dashboard, branding. All members can post; pull up a chair.
 - See the [Charter](charter.md) for how governance is structured.
 - See [Open questions](open-questions.md) for the decisions currently on the table.