helpful note about posting with CORS (#938)

examples/cors/src/main.rs

@@ -24,6 +24,10 @@ async fn main() {
         let app = Router::new().route("/json", get(json)).layer(
             // see https://docs.rs/tower-http/latest/tower_http/cors/index.html
             // for more details
+            //
+            // pay attention that for some request types like posting content-type: application/json
+            // it is required to add ".allow_headers(vec![http::header::CONTENT_TYPE])"
+            // or see this issue https://github.com/tokio-rs/axum/issues/849
             CorsLayer::new()
                 .allow_origin(Origin::exact("http://localhost:3000".parse().unwrap()))
                 .allow_methods(vec![Method::GET]),