Daemon configuration service file path fix and info for Firefox Flatpak

[luca-demma]

In the daemon configuration readme is written to create a file in
/etc/systemd/fx_cast.service
I think it should be in
/etc/systemd/system/fx_cast.service
otherwise the systemctl command fails because it doesn't find the file (tested on Pop_OS (ubuntu based) )

Another improvement IMO would be to add in the homepage the same info about the firefox snap package for the Flatpak package that suffers of the same problem

[Matt Hensman]

Fixed, thanks.