Add readme for web provider

Also rename sample web provider implemented in Rust to `web-rust`.

providers/web/web-rust/src/main.rs

@@ -0,0 +1,20 @@
+extern crate env_logger;
+extern crate kube_rust;
+#[macro_use]
+extern crate log;
+extern crate time;
+extern crate virtual_kubelet_adapter;
+
+mod utils;
+mod unit_provider;
+
+use virtual_kubelet_adapter::start_server;
+use unit_provider::UnitProvider;
+
+fn main() {
+  // initialize logger
+  env_logger::init().unwrap();
+
+  let provider = Box::new(UnitProvider::new());
+  start_server(provider).unwrap();
+}