modify Readme fix print log

Author: Liuchy1

README.md

@@ -256,7 +256,7 @@
             RegisterInfo[] regUsers = new RegisterInfo[users.size()];
 
             String res = client.registerUsers(users.toArray(regUsers));
-            System.out.println(res);
+            LOG.info(res);
         } catch (APIConnectionException e) {
             LOG.error("Connection error. Should retry later. ", e);
         } catch (APIRequestException e) {
@@ -274,7 +274,7 @@
         JMessageClient client = new JMessageClient(appkey, masterSecret);
         try {
             String res = client.createGroup("test_user", "test_gname1", "description", "test_user");
-            System.out.println(res);
+            LOG.info(res);
         } catch (APIConnectionException e) {
             LOG.error("Connection error. Should retry later. ", e);
         } catch (APIRequestException e) {