Update for Solus instructions

[EbonJaeger]

Earlier this year, Solus enabled Flathub by default in the flatpak package, which is also installed by default. While the current Software Center still does not have flatpak support, the team is planning on switching to GNOME Software and KDE Discover, both of which do have integrated flatpak support, and users can install those applications right now.

I was looking at updating the instructions on the webpage myself, but the readme states to make in issue before a pull request, so I figured I'd start with this.

[razzeee]

This needs to be addressed in the other repo https://github.com/flatpak/flatpak.github.io

[razzeee]

Then we can sync it here

[EbonJaeger]

Ah, I see, sorry. Do I need to make an issue over there first, or go straight to pull request once I've made changes?

[razzeee]

PR should be fine there