Add deprecation notice for combined repository

README.md

@@ -1,7 +1,13 @@
 HTTP Server Middleware
 ======================
 
+**Note: This repository is deprecated and no longer updated. Please refer to
+[http-interop/http-server-handler][http-server-handler] and
+[http-interop/http-server-middleware][http-server-middleware] instead.**
+
 This is an implementation of the proposed [PSR-15][psr-15]. Please refer to the
 proposal for a description.
 
 [psr-15]: https://github.com/php-fig/fig-standards/tree/master/proposed/http-middleware
+[http-server-handler]: https://github.com/http-interop/http-server-handler
+[http-server-middleware]: https://github.com/http-interop/http-server-middleware