Update README.md before archiving repo (#78)

Author: jhaaaa

README.md

@@ -1,6 +1,13 @@
-## XMTP Postal Service (XPS) Gateway
+![Status](https://img.shields.io/badge/Deprecated-brown)
+
+> [!CAUTION]
+> This repo is no longer maintained.
+
+The documentation below is provided for historical reference only.
 
-![XPS](xps.png)
+---
+
+## XMTP Postal Service (XPS) Gateway
 
 This _postal service_ gateway implements the XMTP transport for registration, inbox and conversations.