Added comment explaining stdin as a file descriptor

src/main.rs

@@ -357,7 +357,7 @@ fn main() -> Result<()> {
     let mut buf = Vec::new();
 
     match argv.get(1).map(|s| s.as_str()) {
-        Some("-") | None => unsafe { File::from_raw_fd(0) },
+        Some("-") | None => unsafe { File::from_raw_fd(0) }, // stdin as a file
         Some(path) => {
             std::fs::File::open(path).unwrap_or_else(|_| {
                 eprintln!(