Add notice to README

Author: rubengees

README.md

@@ -1,5 +1,9 @@
 # ![](art/logo/proxer-logo-title.png) Proxer.Me Android [![Latest Release](https://img.shields.io/github/release/proxer/ProxerAndroid.svg)](https://github.com/proxer/ProxerAndroid/releases/latest) [![Build status](https://github.com/proxer/ProxerAndroid/workflows/CI/badge.svg)](https://github.com/proxer/ProxerAndroid/actions?workflow=CI)
 
+### Unmaintained
+
+This app is not maintained anymore and will not receive any more updates. It might or might not work anymore.
+
 ### What is this?
 
 Proxer.Me Android is the official mobile client for the german Anime & Manga page [Proxer.Me](https://proxer.me).<br>