message: Add references to transaction serialization format
diff: @@ -45,6 +45,8 @@ fn serialize_unsigned_transaction(
  data: &[u8],
  chain_id: u64,
  ) -> Vec<u8> {
  + // See https://ethereum.stackexchange.com/a/2097/54581 and
  + // https://github.com/tomusdrw/jsonrpc-proxy/blob/7855dec/ethereum-proxy/plugins/accounts/transaction/src/lib.rs#L132-L144.
  let mut stream = RlpStream::new();
  stream.begin_list(9);
  stream.append(&nonce);
  