mention the openssl callbacks for SSL multithread

#include <pthread.h>

#include <curl/curl.h>

/* silly list of test-URLs */

/*

  List of URLs to fetch.

  If you intend to use a SSL-based protocol here you MUST setup the OpenSSL

  callback functions as described here:

  http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION

*/

char *urls[]= {

  "http://curl.haxx.se/",

  "ftp://cool.haxx.se/",

  CURL *curl;

  curl = curl_easy_init();

  curl_easy_setopt(curl, CURLOPT_URL, url);

  curl_easy_perform(curl);

  curl_easy_perform(curl); /* ignores error */

  curl_easy_cleanup(curl);

  return NULL;