Add Flathub to list of unofficial Linux packages

[ssokolow]

Since the Linux section of https://mpv.io/installation/ says all the offered packages are unofficial, it'd probably be a good idea to include https://flathub.org/apps/details/io.mpv.Mpv as well.

It'll be a suitable option for a lot of distros.

[sfan5]

see discussion here #103 (comment)

I created a pull request to add it.