add simplified file input example to readme

Author: cammeresi

README.md

@@ -16,10 +16,37 @@ parse ADIF data in Rust on top of tokio.
 
 ## Usage
 
+Add the dependency:
+
 ```sh
 cargo add TBD
 ```
 
+Then start reading ADIF from any object that implements the AsyncRead trait:
+
+```rust,no_run
+# use adif::RecordStream;
+# use futures::StreamExt;
+# use tokio::fs::File;
+# use tokio::io::BufReader;
+#
+# #[tokio::main(flavor = "current_thread")]
+# async fn main() -> Result<(), Box<dyn std::error::Error>> {
+#     let path = "";
+let file = File::open(path).await?;
+let reader = BufReader::new(file);
+let mut stream = RecordStream::new(reader, true);
+
+while let Some(result) = stream.next().await {
+    let record = result?;
+    if let Some(call) = record.get("call") {
+        println!("call: {}", call.as_str());
+    }
+}
+#
+#     Ok(())
+# }
+```
 See examples or documentation for further information.
 
 ## Features