Document how to provide extra storage

    Hi,

    I was perusing threads yesterday because I want to attach a USB hard drive to my RPI3 to provide extra storage. Specifically, for TVHeadend recordings.

    I found a few threads that hinted that LibreElec would automatically mount filesystems, following the partition label, but I didn't get that to work.

    I also found https://wiki.libreelec.tv/how-to/mount_network_share .

    I am happy to edit and contribute to the wiki:

    * If automatic mounting is not possible, I can edit this page so that it's not limited to "network storage", and explain how to create a systemd unit to mount an external USB HDD, if that is the way to go.

    * If automatic mounting is possible, I can edit this page or create a new one explaining how to get it to work, if I get some hints to get it working.

    Cheers,

    Álex