client.trade().testNewOrder(parameters); not exists

please fix your readme.

`client.trade().testNewOrder(parameters);` is not exists. it is not even in base interface of FuturesClient

```
public interface FuturesClient {
Market market();

Account account();

UserData userData();

PortfolioMargin portfolioMargin();
}
```

Could you please provide proper example of placing new order?

Start with the README and compare its client.trade().testNewOrder(parameters) example with the FuturesClient interface shown in the issue. Identify the valid client entry point for placing a new order and update the example accordingly. Done means the README no longer references the nonexistent method and shows a proper new-order example.

Written by the indexing model from the issue text.