add docs for rust example (#398)

examples/functions/Rust/README.md

@@ -0,0 +1,47 @@
+# Make a Rust function a service with fx
+
+Write a function like,
+
+```rust
+pub mod fns {
+    #[derive(Serialize)]
+    pub struct Response {
+        pub result: i32,
+    }
+
+    #[derive(Deserialize)]
+    pub struct Request {
+        pub a: i32,
+        pub b: i32,
+    }
+
+    pub fn func(req: Request) -> Response {
+        Response {
+            result: req.a + req.b,
+        }
+    }
+}
+
+```
+
+then deploy it with `fx up` command,
+
+```shell
+$ fx up -p 8080 func.rs
+```
+
+test it using `curl`
+
+```shell
+$ curl -X 'POST' --header 'Content-Type: application/json' --data '{"a":1,"b":1}' '0.0.0.0:3000'
+
+HTTP/1.1 200 OK
+Content-Length: 12
+Content-Type: application/json
+Date: Fri, 06 Dec 2019 06:45:14 GMT
+Server: Rocket
+
+{
+    "result": 2
+}
+```