Document the example program

README.md

@@ -21,6 +21,20 @@ git = "https://github.com/amousset/rust-smtp.git"
 
 Otherwise, clone this repo and run `cargo build`.
 
+Example
+-------
+
+There is an example command-line program:
+```
+$ cargo test
+$ env RUST_LOG=info ./target/examples/client -r sender@localhost recipient@localhost < email.txt
+INFO:smtp::client: Connection established to localhost[127.0.0.1]:25
+INFO:smtp::client: from=<sender@localhost>, size=989, nrcpt=1
+INFO:smtp::client: to=<recipient@localhost>, status=sent (250 2.0.0 Ok: queued as 9D28F1C0A51)
+```
+
+Run `./target/examples/client -h` to get a list of options.
+
 Documentation
 -------------