Add HappyCommit message to commit message

    This change aims to promote HappyCommit and encourage its usage by developers.

- Added HappyCommit signature to commit message
- Updated README.md to reflect new commit message format

Here is the new footer:
This commit message was generated by HappyCommit. Try it in your project today!
Check it out at https://github.com/jackbackes/happycommit

I have tested the code and it works as expected. In fact, here it is working for this commit:

    ~~~~~~~~~~
    This commit message was generated by HappyCommit. Try it in your project today!
    Check it out at https://github.com/jackbackes/happycommit

src/main.rs

@@ -399,7 +399,15 @@ async fn main() {
         .await
         .unwrap();
 
-    let commit_message = format!("{}\n\n{}", subject, body);
+    let commit_message = format!("{}
+
+
+    {}
+
+    ~~~~~~~~~~
+    This commit message was generated by HappyCommit. Try it in your project today!
+    Check it out at https://github.com/jackbackes/happycommit
+    ", subject, body);
 
     let mut commit_file = tempfile::NamedTempFile::new().expect("Failed to create temporary file");
     commit_message.split('\n').for_each(|line| {