Method taken from https://github.com/pimusicbox/pimusicbox
Assuming mopidy and Iris etc are already installed, we need to install mpc, so that we can stop playback on mopidy when we use spotify connect
sudo apt-get install mpc
Then we install the easiest librespot spotify connect system
curl -sL https://dtcooper.github.io/raspotify/install.sh | sh
Then we just need to create a script to stop mpd playback, and customise raspotify to call this script when we start playback.