Mellite @ Piksel21 - Installation

Corresponding video to https://github.com/Sciss/Mellite-piksel21/blob/main/text/setup.md - installing Mellite ahead of the workshop. For workshop details and registration, see https://21.piksel.no/ --- Erratum: 6'24" on macOS, you probably put your SuperCollider application into the `/Applications` directory (instead of `/Application`) and the bundle is `SuperCollider.app` instead of just `SuperCollider`.