Update README to indicate repository is archived

Added an archived notice and link to Synapse as the preferred implementation.

Author: hughns

README.md

@@ -1,5 +1,12 @@
 # Node.js HTTP Rendezvous Server
 
+## This repository is archived
+
+[Synapse](https://github.com/element-hq/synapse) has this capability built in since v1.106.0 and is the preferred implementation. Hence this repository is now archived.
+
+n.b. This repository implemented the [2024 version of MSC4108](https://github.com/matrix-org/matrix-spec-proposals/blob/87f8317a902cd7bc5c2d2d225f71021b3a509e2d/proposals/4108-oidc-qr-login.md) which will be superseded.
+
+---
 A standalone implementation of the rendezvous session API proposed by [MSC4108: Mechanism to allow OIDC sign in and E2EE set up via QR code](https://github.com/matrix-org/matrix-spec-proposals/pull/4108).
 
 Functionality constraints: