Document persistent cert volume for mumble bridge

mumble-bridge/README.md

@@ -84,3 +84,41 @@ For automatic self-signed cert generation when files are missing:
       # optional:
       # MUMBLE_CERT_SUBJECT: "/CN=EVS-esp32-evs-1"
 ```
+
+## Persistent cert volume (recommended)
+
+If `MUMBLE_AUTO_CERT=true`, mount `/data` as a persistent volume.
+Otherwise a new cert may be generated after redeploy and Mumble can reject login for an already-registered username.
+
+Example:
+
+```yaml
+services:
+  evs-mumble-esp32-1:
+    image: git.khnm-zimmerling.de/kai/evs-mumble-bridge:latest
+    restart: unless-stopped
+    ports:
+      - "5004:5004/udp"
+    environment:
+      DEVICE_ID: "esp32-evs-1"
+      MUMBLE_HOST: "10.100.3.50"
+      MUMBLE_PORT: "64738"
+      MUMBLE_USERNAME: "EVSClient1"
+      MUMBLE_AUTO_CERT: "true"
+      MUMBLE_CERT_DIR: "/data/certs"
+      MUMBLE_CERT_AUTO_RENEW: "true"
+      MUMBLE_CERT_RENEW_BEFORE_DAYS: "30"
+      UDP_LISTEN_HOST: "0.0.0.0"
+      UDP_LISTEN_PORT: "5004"
+      INPUT_SAMPLE_RATE: "16000"
+    volumes:
+      - evs_mumble_esp32_1_data:/data
+
+volumes:
+  evs_mumble_esp32_1_data:
+```
+
+If the user is already registered with another cert, either:
+- provide the original `MUMBLE_CERTFILE`/`MUMBLE_KEYFILE`, or
+- use a new `MUMBLE_USERNAME`, or
+- re-register that username on the Mumble server.