message: doc: add async example to README.md
diff: @@ -46,15 +46,16 @@ use cdrs::cluster::{ClusterTcpConfig, NodeTcpConfigBuilder};
  use cdrs::load_balancing::RoundRobin;
  use cdrs::query::*;
  +#[tokio::main]
  fn main() {
  let node = NodeTcpConfigBuilder::new("127.0.0.1:9042", NoneAuthenticator {}).build();
  let cluster_config = ClusterTcpConfig(vec![node]);
  let no_compression =
  - new_session(&cluster_config, RoundRobin::new()).expect("session should be created");
  + new_session(&cluster_config, RoundRobin::new()).await.expect("session should be created");
  let create_ks: &'static str = "CREATE KEYSPACE IF NOT EXISTS test_ks WITH REPLICATION = { \
  'class' : 'SimpleStrategy', 'replication_factor' : 1 };";
  - no_compression.query(create_ks).expect("Keyspace create error");
  + no_compression.query(create_ks).await.expect("Keyspace create error");
  }
  ```
  