Add a donate button to README.md.

README.md

@@ -1,7 +1,8 @@
-# Mixxx 
+# Mixxx
 [![GitHub latest tag](https://img.shields.io/github/tag/mixxxdj/mixxx.svg)](https://www.mixxx.org/download)
 [![Packaging status](https://repology.org/badge/tiny-repos/mixxx.svg)](https://repology.org/metapackage/mixxx/versions)
 [![Zulip chat](https://img.shields.io/badge/zulip-join_chat-brightgreen.svg)](https://mixxx.zulipchat.com)
+[![Donate](https://img.shields.io/badge/Donate-PayPal-green.svg)](https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=QSFMYWN2B3JD2&source=url)
 
 [Mixxx] is Free DJ software that gives you everything you need to perform live
 DJ mixes. Mixxx works on GNU/Linux, Windows, and macOS.