Updated readme to highlight compatability with old API

Author: confuzzledduck

README.md

@@ -1,7 +1,9 @@
 convertapi-php
 ==============
 
-  PHP library for the [convertapi.com](http://www.convertapi.com) REST API.
+PHP library for the [convertapi.com](http://www.convertapi.com) REST API.
+
+NB. This library currently works with the **old** convertapi API, **not** the newer (v2) API.
 
 ConvertApi is an online file conversion service for creating PDF and Images from various source files, such as Word documents, web pages and raw HTML. converapi-php provides a set of classes for using the ConvertApi service with PHP.