Add link to matrix space on main page

index.md

@@ -22,6 +22,7 @@ systems such as Windows in the near future.
 * Source code: <https://gitlab.freedesktop.org/monado/monado>
 * GSoC 2023: [Proposal guide](https://monado.freedesktop.org/gsoc-proposal-guide.html)
   and [project ideas](https://monado.freedesktop.org/gsoc-2023.html).
+* Matrix space: <https://matrix.to/#/#monado:matrix.org>
 * Discord server: <https://discord.gg/8RkJgRJ>
 * IRC channel: [#monado](https://webchat.oftc.net/?channels=monado) on
   [OFTC](https://oftc.net/)