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