docs(readme): document --relay / PIXELPASS_RELAY

Add a Relay section covering the flag and env-var forms, precedence,
the GUI-child forwarding, and the same-relay-on-both-ends requirement.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -223,6 +223,25 @@ measured_at = "2026-05-21T20:41:16Z"
 - Skip is sticky — once you skip the test, pixelpass won't ask again
   unless you reconfigure.
 
+## Relay
+
+By default pixelpass uses iroh's bundled relay servers to coordinate the
+P2P connection (peers still hole-punch a direct UDP path when they can; the
+relay is the fallback and the rendezvous point). You can point it at a
+different relay — a self-hosted one, or n0's staging/production servers —
+with either:
+
+```bash
+pixelpass --relay https://relay.example/            # host or viewer
+PIXELPASS_RELAY=https://relay.example/ pixelpass …  # env-var form
+```
+
+The flag applies to both host and viewer and takes precedence over the
+environment variable. The env-var form is handy for the `--gui` front-end,
+since the GUI's child host/viewer processes inherit it; the `--gui --relay`
+flag form is forwarded to them too. Both ends must use the same relay to
+find each other.
+
 ## Audio
 
 By default pixelpass captures the default sink's monitor — the viewer