Archive repository and update README

Added archiving notice and links to superseded MSC proposals.

Author: hughns

README.md

@@ -1,5 +1,10 @@
 # Rust HTTP Rendezvous Server
 
+## This repository is archived
+
+This repository implemented [MSC3886](https://github.com/matrix-org/matrix-spec-proposals/pull/3886) which has been closed and superseded by [MSC4108](https://github.com/matrix-org/matrix-spec-proposals/pull/4108). As such, this repository is now archived.
+
+---
 An implementation of [MSC3886: Simple rendezvous capability](https://github.com/matrix-org/matrix-spec-proposals/pull/3886) that can be used standalone or packaged as a Synapse module.
 
 Functionality constraints: