Update README

Add note to the top of the README to explain what happened to serf.io

README.md

@@ -1,5 +1,12 @@
 # Serf [![Build Status](https://github.com/hashicorp/serf/workflows/Checks/badge.svg)](https://github.com/hashicorp/serf/actions) [![Join the chat at https://gitter.im/hashicorp-serf/Lobby](https://badges.gitter.im/hashicorp-serf/Lobby.svg)](https://gitter.im/hashicorp-serf/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
+
+> [!NOTE]
+> Looking for serf.io? The Serf website was shut down on 10/02/2024. The docs
+previously served from serf.io can be found
+[here](https://github.com/hashicorp/serf/blob/master/docs/index.html.markdown)
+
+
 * Website: https://github.com/hashicorp/serf
 * Chat: [Gitter](https://gitter.im/hashicorp-serf/Lobby)
 * Mailing list: [Google Groups](https://groups.google.com/group/serfdom/)