add uds example readme

examples/unix-socket/README.md

@@ -0,0 +1,14 @@
+## Unix domain socket example
+
+```bash
+$ curl --unix-socket /tmp/actix-uds.socket http://localhost/
+Hello world!
+```
+
+Although this will only one thread for handling incoming connections 
+according to the 
+[documentation](https://actix.github.io/actix-web/actix_web/struct.HttpServer.html#method.start_incoming).
+
+And it does not delete the socket file (`/tmp/actix-uds.socket`) when stopping
+the server so it will fail to start next time you run it unless you delete
+the socket file manually.