Add deprecation notice

Author: alexcrichton

README.md

@@ -1,3 +1,8 @@
+> **Note**: This repository and organization are archived and this is [no longer
+> where activity for Rust and
+> WebAssembly](https://blog.rust-lang.org/inside-rust/2025/07/21/sunsetting-the-rustwasm-github-org/)
+> is happening.
+
 # Rust and WebAssembly Landing Page
 
 This repository contains the source of https://rustwasm.github.io, the main