Create a warning for OpenSUSE and NixOS due to low package availability

> Mullvad Browser is packaged as a flatpak This may be the case but generally it is recommended to not install browsers as flatpaks because it can negatively affect their built-in sandboxing. Additionally the Mullvad Browser flatpaks is unofficial, meaning it is not maintained by the folks at Mullvad and you’ll have to trust the maintainers of this flatpak (like you have to trust Linux distribution repository maintainers). I don’t have OpenSuse installed, though I could potentially spin up a VM of it later to test the MullvadVPN package using Fedora installation files/instructions.