Demonstrate how to get help center articles by section_id (#536)

- We need to wrap the client in a help center resource, otherwise the request
  will not have what it needs to retrieve all articles, rather than a given
  section
- Provide a specific example demonstrating how to list articles by section_id

samples/helpcenter/findArticles.php

@@ -14,10 +14,11 @@
 
 $client = new ZendeskAPI($subdomain);
 $client->setAuth('basic', ['username' => $username, 'token' => $token]);
+$help_center_client = new Zendesk\API\Resources\HelpCenter($client);
 
 try {
     // Find all helpcenter categories
-    $articles = $client->helpCenter->articles()->findAll();
+    $articles = $help_center_client->articles()->findAll();
 
     echo "<pre>";
     print_r($articles);

samples/helpcenter/findArticlesBySectionId.php

@@ -0,0 +1,29 @@
+<?php
+
+require_once __DIR__ . '/../../vendor/autoload.php';
+
+use Zendesk\API\HttpClient as ZendeskAPI;
+
+/**
+ * Replace the following with your own.
+ */
+
+$subdomain = "subdomain";
+$username = "email@example.com";
+$token = "6wiIBWbGkBMo1mRDMuVwkw1EPsNkeUj95PIz2akv";
+$section_id = 10801195364239; // replace this with your section id
+
+$client = new ZendeskAPI($subdomain);
+$client->setAuth('basic', ['username' => $username, 'token' => $token]);
+$help_center_client = new Zendesk\API\Resources\HelpCenter($client);
+
+try {
+    // Find all helpcenter category with the given section id
+    $articles = $help_center_client->sections($section_id)->articles()->findAll();
+
+    echo "<pre>";
+    print_r($articles);
+    echo "</pre>";
+} catch (\Zendesk\API\Exceptions\ApiResponseException $e) {
+    echo $e->getMessage().'</br>';
+}