added example for slack notification dev feature

examples/slack-notification.php

@@ -0,0 +1,26 @@
+<?
+
+require 'vendor/autoload.php';
+
+use \TANIOS\Airtable\Airtable;
+
+$airtable = new Airtable(array(
+    'api_key' => 'API_KEY',
+    'base'    => 'BASE_ID',
+    'slack_webhook' => 'SLACK_WEBHOOK_URL' // Get the URL here: https://api.slack.com/incoming-webhooks
+));
+
+
+$images_array = array(
+	array(
+	 	'url' => 'IMAGE_URL'
+	)
+);
+
+$update_contact_details = array(
+	'Image' => $images_array // test by changing the field name to something that doesn't exist
+);
+
+$update_contact = $airtable->updateContent("Customers/{entry-id}",$update_contact_details);
+
+print_r($update_contact);