Add flathub flatpak mpv to Installation instructions

Author: fastrizwaan

source/installation.html.haml

@@ -68,6 +68,8 @@ title: Installation
             Distributions usually package outdated, unmaintained, and unsupported versions of mpv. This is especially
             true for popular distros like Debian and Ubuntu. You are recommended to use
             mpv-build or third-party packages instead. All of these packages are unofficial.
+      
+      = package_row 'Flatpak (Flathub)', 'https://flathub.org/apps/io.mpv.Mpv'
 
       = package_row 'Arch (official package)', 'https://archlinux.org/packages/extra/x86_64/mpv/'
       = package_row 'Gentoo (official package)', 'https://packages.gentoo.org/packages/media-video/mpv'