docs(README.md): added deprecation notice

Author: KaiSchwarz-cnic

README.md

@@ -5,6 +5,8 @@
 [![License: MIT](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 [![PRs welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](https://github.com/centralnicgroup-opensource/rtldev-middleware-perl-sdk/blob/master/CONTRIBUTING.md)
 
+> DEPRECATION NOTICE: This module received deprecation as of the deprecation of the HEXONET Brand and Platform. We also decided to discontiue it for the CentralNic Reseller Platform as of no customer interest.
+
 This module is a connector library for the insanely fast HEXONET Backend API. For further informations visit our [homepage](http://hexonet.net) and do not hesitate to [contact us](https://www.hexonet.net/contact).
 
 ## Resources