code: public WebhookCluster addWebhooks(WebhookClient... clients)
      {
          Checks.notNull(clients, "Clients");
          for (WebhookClient client : clients)
          {
              Checks.notNull(client, "Client");
              Checks.check(!client.isShutdown,
                  "One of the provided WebhookClients has been closed already!");
              webhooks.add(client);
          }
          return this;
      }
docs: Adds the specified {@link net.dv8tion.jda.webhook.WebhookClient WebhookClients}
  to this cluster's list of receivers.
  <br>Duplicate clients are supported and will not be filtered automatically.
  
  @param  clients
  WebhookClients to add
  
  @throws java.lang.IllegalArgumentException
  If the provided array or any of the contained
  clients is {@code null} or closed
  
  @return The current WebhookCluster for chaining convenience