Add Slackware instructions

[britalmeida]

My distribution (Slackware) is not in the website to follow instructions.
You have no path to follow in case a distribution is not listed.

[AsciiWolf]

I see that Flatpak is available in the SlackBuilds repository, but the version is quite old.

[cole-rogers]

I would like to add support for this and suggest adding build and install instructions for Slackware, since Flatpak is probably pretty attractive to Slackware users. Although, they also probably know how to compile Flatpak and install it anyways. Still, I think they would appreciate not having to hunt down the github page, find the readme, and the look at the hacking section. Could also be added as just a generic build from tarball button too.

Other niche distros like PC Linux OS could probably benefit from this as well.

[tooomm]

Regarding distrowatch.com with their ranking as indicator for famous distros...

Flathub misses "Manjaro" and "MX Linux" as well, both on top of the list at distrowatch!