Update README.md (#2770)

* Update README.md

Quick helper on the async_std quickstart

* Move comment regard. `attributes` into code example

* doc(README): adjust wording of a comment

---------

Co-authored-by: Austin Bonander <austin.bonander@gmail.com>

README.md

@@ -234,7 +234,7 @@ use sqlx::postgres::PgPoolOptions;
 // use sqlx::mysql::MySqlPoolOptions;
 // etc.
 
-#[async_std::main]
+#[async_std::main] // Requires the `attributes` feature of `async-std`
 // or #[tokio::main]
 // or #[actix_web::main]
 async fn main() -> Result<(), sqlx::Error> {
@@ -257,6 +257,7 @@ async fn main() -> Result<(), sqlx::Error> {
 }
 ```
 
+
 ### Connecting
 
 A single connection can be established using any of the database connection types and calling `connect()`.