message: Explain why top comments are preserved in Cargo.lock
diff: @@ -43,6 +43,9 @@ pub fn write_pkg_lockfile(ws: &Workspace, resolve: &Resolve) -> CargoResult<()>
  let mut out = String::new();
  + // Preserve the top comments in the lockfile
  + // This is in preparation for marking it as generated
  + // https://github.com/rust-lang/cargo/issues/6180
  if let Ok(orig) = &orig {
  for line in orig.lines().take_while(|line| line.starts_with("#")) {
  out.push_str(line);
  