Add auto-reload example (#2166)

examples/auto-reload/Cargo.toml

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+[package]
+name = "auto-reload"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+axum = { path = "../../axum" }
+listenfd = "1.0.1"
+tokio = { version = "1.0", features = ["full"] }

examples/auto-reload/README.md

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+# auto-reload
+
+This example shows how you can set up a development environment for your axum
+service such that whenever the source code changes, the app is recompiled and
+restarted. It uses `listenfd` to be able to migrate connections from an old
+version of the app to a newly-compiled version.
+
+## Setup
+
+```sh
+cargo install cargo-watch systemfd
+```
+
+## Running
+
+```sh
+systemfd --no-pid -s http::3000 -- cargo watch -x run
+```

examples/auto-reload/src/main.rs

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+//! Run with
+//!
+//! ```not_rust
+//! cargo run -p example-hello-world
+//! ```
+
+use axum::{response::Html, routing::get, Router};
+use listenfd::ListenFd;
+use tokio::net::TcpListener;
+
+#[tokio::main]
+async fn main() {
+    // build our application with a route
+    let app = Router::new().route("/", get(handler));
+
+    let mut listenfd = ListenFd::from_env();
+    let listener = match listenfd.take_tcp_listener(0).unwrap() {
+        // if we are given a tcp listener on listen fd 0, we use that one
+        Some(listener) => TcpListener::from_std(listener).unwrap(),
+        // otherwise fall back to local listening
+        None => TcpListener::bind("127.0.0.1:3000").await.unwrap(),
+    };
+
+    // run it
+    println!("listening on {}", listener.local_addr().unwrap());
+    axum::serve(listener, app).await.unwrap();
+}
+
+async fn handler() -> Html<&'static str> {
+    Html("<h1>Hello, World!</h1>")
+}