Matrix: add user creation instructions

Matrix.md

@@ -8,6 +8,31 @@ Homeserver: `mtx.alogins.net` (Caddy → Synapse container port 8008)
 
 Compose directory: `agap_git/matrix/`
 
+## Creating New Users
+
+Registration is disabled by default. Use `register_new_matrix_user` with the shared secret from `homeserver.yaml`.
+
+```bash
+docker exec -it matrix-synapse-1 register_new_matrix_user \
+  -u <username> \
+  -p <password> \
+  -c /data/homeserver.yaml \
+  http://localhost:8008
+```
+
+Add `--admin` flag to create an admin user. The shared secret is in `~/agap_git/matrix/data/synapse/homeserver.yaml` under `registration_shared_secret`.
+
+To create a user non-interactively:
+
+```bash
+docker exec matrix-synapse-1 register_new_matrix_user \
+  -u <username> \
+  -p <password> \
+  --no-admin \
+  -c /data/homeserver.yaml \
+  http://localhost:8008
+```
+
 ## Matrix Bot
 
 Repo: `~/matrixbot/` — `http://localhost:3000/alvis/matrixbot` (if pushed)