Add Dockerfile and Docker Hub publishing (#9)

Adds docker image publishing with example usage in the README.

src/mail.rs

@@ -49,7 +49,7 @@ pub fn send(topic: &Topic, posts: &Posts, expression: &str, config: &Config) {
         .add_html(&html_message);
 
     match sg.send(mail_info) {
-        Err(err) => println!("Error: {}", err),
-        Ok(body) => println!("Response: {}", body),
+        Err(err) => println!("SendGrid failed to send mail. Error: {}", err),
+        Ok(body) => println!("SendGrid Response: {}", body),
     };
 }