message: add Matrix channel to readme
diff: @@ -7,6 +7,7 @@ https://lumina-desktop.org/
  **Community Chat Channels:**
  - Telegram: ["Lumina Desktop" public channel](https://t.me/luminadesktop)
  + - Matrix: ["Lumina Desktop" public channel on Matrix](#lumina-desktop:matrix.org)
  **Lumina Project Info: Table of Contents**
  