Added "unmaintained" branch message

Author: devgianlu

README.md

@@ -4,6 +4,8 @@
 
 `librespot-java` is a port of [librespot](https://github.com/librespot-org/librespot), originally written in Rust, which as evolved into the most up-to-date open-source Spotify client. Additionally, this implementation provides an useful API to request metadata or control the player, more [here](https://github.com/librespot-org/librespot-java/blob/master/api).
 
+### Only PRs will be accepted for branch `master`, please checkout `new-spotify-api`.
+
 ## Disclaimer!
 We (the librespot-org organization and me) **DO NOT** encourage piracy and **DO NOT** support any form of downloader/recorder designed with the help of this repository. If you're brave enough to put at risk this entire project, just don't publish it. This is meant to provide support for all those devices that are not officially supported by Spotify.