Update Discord link to point to #rules (#3588)

We shuffled the channels around a bit and #welcome got deleted. #rules is what we want to link to now.

README.md

@@ -3,7 +3,7 @@
 [![Build status](https://img.shields.io/travis/LMMS/lmms.svg?maxAge=3600)](https://travis-ci.org/LMMS/lmms)
 [![Latest stable release](https://img.shields.io/github/release/LMMS/lmms.svg?maxAge=3600)](https://lmms.io/download)
 [![Overall downloads on Github](https://img.shields.io/github/downloads/LMMS/lmms/total.svg?maxAge=3600)](https://github.com/LMMS/lmms/releases)
-[![Join the chat at Discord](https://img.shields.io/badge/chat-on%20discord-7289DA.svg)](https://discord.gg/PHcQDx3)
+[![Join the chat at Discord](https://img.shields.io/badge/chat-on%20discord-7289DA.svg)](https://discord.gg/3sc5su7)
 [![Localise on transifex](https://img.shields.io/badge/localise-on_transifex-green.svg)](https://www.transifex.com/lmms/lmms/)
 
 What is LMMS?