docs(contributing): add the invite link to the discord server (#570)

CONTRIBUTING.md

@@ -3,7 +3,7 @@
 Thanks for considering to contribute to Zellij!
 
 **First**: if you're unsure or afraid of anything, just ask on our [Discord
-server][discord-invite-link] or submit the issue or pull request anyway. You
+server](https://discord.gg/MHV3n76PDq) or submit the issue or pull request anyway. You
 won't be yelled at for giving it your best effort. The worst that can happen is
 that you'll be politely asked to change something. We appreciate any sort of
 contributions, and don't want a wall of rules to get in the way of that.