docs: document host UDP buffer sysctl tuning

LiveKit's startup warning about UDP receive buffer being too small
(default 212992 bytes vs suggested 5000000) is easy to overlook, but
under load — especially when many clients are forced through the TURN
relay — it causes dtls timeouts on publisher transports and asymmetric
black-tile / no-media symptoms. The fix has to be on the host because
net.core.rmem_max / wmem_max are read when LiveKit opens its UDP sockets
and can't be raised from inside the container.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -103,6 +103,39 @@ This recipe publishes ports directly on the host for WebRTC media transport. The
 
 Your firewall must allow inbound traffic on these ports.
 
+### Host kernel tuning
+
+LiveKit logs a warning at startup if the kernel's UDP socket buffers are too small:
+
+```
+WARN livekit rtcconfig/rtc_unix.go:31 UDP receive buffer is too small for a production set-up {"current": 425984, "suggested": 5000000}
+```
+
+The Linux default (`net.core.rmem_max = 212992`) is well under what LiveKit needs once
+several participants are forced through the TURN relay path. The resulting packet
+loss shows up as `dtls timeout: read/write timeout: context deadline exceeded` on
+publisher transports, intermittent media stalls, or one peer seeing a black tile
+while the other sees video.
+
+These sysctls are read by LiveKit when it opens its UDP sockets, so they must be
+set on the **host** (not in the container) before the LiveKit container starts.
+
+On the host, create `/etc/sysctl.d/99-livekit.conf`:
+
+```
+net.core.rmem_max = 7500000
+net.core.wmem_max = 7500000
+```
+
+Then apply and restart the service:
+
+```
+sudo sysctl --system
+docker service update --force <stack>_livekit
+```
+
+The warning should be gone from the LiveKit boot log.
+
 ### TURN server
 
 TURN is enabled by default and helps users behind CGNAT/symmetric NAT connect to video calls. To disable it, remove `compose.turn.yml` from `COMPOSE_FILE` in your app config and set `LIVEKIT_TURN_ENABLED=false`.