Add install prebuild flatpak to readme

[freddii]

sudo apt-get install flatpak -y
flatpak remote-add --user --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
flatpak install --user io.github.mightycreak.Diffuse -y

[MightyCreak]

It's actually even easier now that the app has been approved, just this will work (if you already have flathub installed):

flatpak install io.github.mightycreak.Diffuse

Or you can use GNOME Software:

[MightyCreak]

But I agree with you, I should add more instructions in the readme!