readme: add fn main to getting started example (#990)

README.md

@@ -71,27 +71,29 @@ use lettre::message::header::ContentType;
 use lettre::transport::smtp::authentication::Credentials;
 use lettre::{Message, SmtpTransport, Transport};
 
-let email = Message::builder()
-    .from("NoBody <nobody@domain.tld>".parse().unwrap())
-    .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
-    .to("Hei <hei@domain.tld>".parse().unwrap())
-    .subject("Happy new year")
-    .header(ContentType::TEXT_PLAIN)
-    .body(String::from("Be happy!"))
-    .unwrap();
+fn main() {
+    let email = Message::builder()
+        .from("NoBody <nobody@domain.tld>".parse().unwrap())
+        .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
+        .to("Hei <hei@domain.tld>".parse().unwrap())
+        .subject("Happy new year")
+        .header(ContentType::TEXT_PLAIN)
+        .body(String::from("Be happy!"))
+        .unwrap();
 
-let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
+    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());
 
-// Open a remote connection to gmail
-let mailer = SmtpTransport::relay("smtp.gmail.com")
-    .unwrap()
-    .credentials(creds)
-    .build();
+    // Open a remote connection to gmail
+    let mailer = SmtpTransport::relay("smtp.gmail.com")
+        .unwrap()
+        .credentials(creds)
+        .build();
 
-// Send the email
-match mailer.send(&email) {
-    Ok(_) => println!("Email sent successfully!"),
-    Err(e) => panic!("Could not send email: {e:?}"),
+    // Send the email
+    match mailer.send(&email) {
+        Ok(_) => println!("Email sent successfully!"),
+        Err(e) => panic!("Could not send email: {e:?}"),
+    }
 }
 ```