doc: replace IRC link with Discord (#5019)

.github/CONTRIBUTING.md

@@ -4,18 +4,22 @@ If you would like to contribute code, documentation, or other assets you can do
 
 *When submitting code, please make every effort to follow existing conventions and style in order to keep the code as readable as possible.*
 
-Read on for an overview and [check the wiki for more details](https://github.com/MovingBlocks/Terasology/wiki). For questions please join us in our [forum](http://forum.terasology.org/forum/) or on `#terasology` (irc.freenode.net).
+Read on for an overview and [check the wiki for more details](https://github.com/MovingBlocks/Terasology/wiki).
+For questions please join us in our [forum] or [Discord].
 
 ## File an Issue
 
 You can report bugs and feature requests to [GitHub Issues](https://github.com/MovingBlocks/Terasology/issues). As mentioned please look for a similar existing issue before submitting a new one.
 
 For finding easy to do issues to start with look at the [Good First Issue](https://github.com/MovingBlocks/Terasology/labels/Good%20First%20Issue) issues.
 
-We prefer questions and support requests be posted in the [forum](http://forum.terasology.org/forum).
+We prefer questions and support requests be posted in the [forum] or [Discord].
 
 __Please provide as much information as possible to help us solve problems and answer questions better!__
 
+[forum]: https://forum.terasology.org/forum/
+[Discord]: https://discord.gg/Terasology
+
 ## PR Title / Commit Message Guidelines
 
 We try to follow the [conventional commits](https://www.conventionalcommits.org/en/v1.0.0-beta.2/) style for commit messages and pull request titles.