Describe the method for getting the value of the cluster ports in README

[MaratKarimov]

This will be useful for testing REST API

[MaratKarimov]

Example of REST API test

import org.junit.ClassRule;
import org.junit.Test;
import org.junit.rules.ExternalResource;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.testcontainers.containers.GenericContainer;
import org.testcontainers.containers.TarantoolCartridgeContainer;

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;

public class AppTest {

    private final Logger logger = LoggerFactory.getLogger(AppTest.class);

    private static final GenericContainer<?> container =
            new TarantoolCartridgeContainer("cartridge/instances.yml", "cartridge/topology.lua")
                    .withDirectoryBinding("cartridge")
                    .withRouterHost("localhost")
                    .withRouterPort(3301)
                    .withAPIPort(8081)
                    .withRouterUsername("admin")
                    .withRouterPassword("tarantool-cartridge-starter-cluster-cookie")
                    .withReuse(true);

    @ClassRule
    public static ExternalResource resource = new ExternalResource() {
        @Override
        public void before() {
            container.start();
        }
        @Override
        public void after() {
            container.stop();
        }
    };


    @Test
    public void shouldAnswerWithTrue() throws IOException {
        final int mappedHttpPort = container.getMappedPort(8081);
        final String metricsResponse = sendRequestAndGetResponse("http://localhost:" + mappedHttpPort + "/metrics");
        logger.info("Metric response: ", metricsResponse);
        final String helloResponse = sendRequestAndGetResponse("http://localhost:" + mappedHttpPort + "/hello");
        logger.info("Hello response: ", helloResponse);
    }

    private final String sendRequestAndGetResponse(final String urlSource) throws IOException {
        final URL url = new URL(urlSource);
        final HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        return connection.getResponseMessage();
    }
}