docs(readme): document the stdin option

Refs #2

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TWLFEoRaRafJm1SpdhWQ6F

README.md

@@ -47,6 +47,25 @@ Waiting does **not** consume the restart budget: a slow dependency costs
 patience, not retries. This is what stops a Docker-backed server from being
 marked failed at login while the Docker daemon is still starting.
 
+## Servers that read stdin
+
+MCP servers are stdio-first by convention, and several that also speak HTTP
+still start their stdio transport unconditionally. Under the launchd agent the
+daemon's stdin is `/dev/null`, so such a server sees EOF on its first read and
+shuts down seconds after binding its port.
+
+`stdin` picks what the child gets on fd 0:
+
+    stdin "keep-open"
+
+- `inherit` (default) — the child inherits the daemon's stdin.
+- `null` — `/dev/null`; a read sees EOF immediately.
+- `keep-open` — a pipe that `xy` holds open for the child's lifetime and never
+  writes to, so a read blocks instead of seeing EOF.
+
+Use `keep-open` for a server you want supervised in HTTP mode that insists on
+running its stdio transport anyway.
+
 ## Start on login (macOS)
 
     xy service install     # write the LaunchAgent, load it, start the daemon