feat: add README.md

__concurrent/async_study/examples/async4.rs

@@ -0,0 +1,10 @@
+fn main() {
+    let rt = tokio::runtime::Runtime::new().unwrap();
+    rt.block_on(async {
+        invoke().await
+    });
+}
+
+async fn invoke() {
+    println!("Hello World!");
+}