readme: Recommend images from flathub #669

Job	Run time
Flatpak Builder Stop At	2m 43s
Flatpak Builder (x86_64, use-cache, cache-restored)	2m 43s
Flatpak Builder (x86_64, use-cache, no-cache-restored)	2m 30s
Tests (18.x)	31s
Tests (20.x)	17s
Flatpak Builder (aarch64, use-cache, cache-restored)	3m 43s
Flatpak Builder (x86_64, no-cache, no-cache-restored)	2m 29s
Lint	26s
Flatpak Builder Cache Hit	2m 56s
	18m 18s

Provide feedback