docs(zero-swift): say Linux needs a socket of your own

Amp-Thread-ID: https://ampcode.com/threads/T-c513a037-a257-45e0-b9a9-15c15edfe859
Co-authored-by: tmm <tmm@tmm.dev>

Author: ampagent

swift/README.md

@@ -15,7 +15,8 @@ end-to-end tests against `zero-cache` and the zbugs app.
 ## Requirements
 
 - Swift 6.2
-- iOS 26 or macOS 26. The `Zero` module also builds on Linux.
+- iOS 26 or macOS 26. The `Zero` module also builds on Linux, without a
+  socket: pass your own `SocketFactory` to sync there.
 - A `zero-cache` that speaks sync protocol 53 (Zero 1.11 or later)
 
 ## Install