Add async/await example. (redis-rs#261)

Add async/await example.

Author: badboy

examples/async-await.rs

@@ -0,0 +1,21 @@
+use redis::AsyncCommands;
+
+#[tokio::main]
+async fn main() -> redis::RedisResult<()> {
+    let client = redis::Client::open("redis://127.0.0.1/").unwrap();
+    let mut con = client.get_async_connection().await?;
+
+    let () = con.set("key1", b"foo").await?;
+
+    let () = redis::cmd("SET")
+        .arg(&["key2", "bar"])
+        .query_async(&mut con)
+        .await?;
+
+    let result = redis::cmd("MGET")
+        .arg(&["key1", "key2"])
+        .query_async(&mut con)
+        .await;
+    assert_eq!(result, Ok(("foo".to_string(), b"bar".to_vec())));
+    Ok(())
+}