[docs] durs: add README in bin crate durs-server

bin/durs-server/README.md

+![Duniter-Rust logo](https://git.duniter.org/nodes/rust/duniter-rs/raw/dev/images/duniter-rs.png)
+
+# Durs-Server
+
+Command line variant for Durs project.
+
+For more informations, see [README at root of project](../../README.md).