README.md: Add clarification note about project scope

Related with #104
8 jobs for adamjones/scope-clarification in 165 minutes and 14 seconds (queued for 2 minutes and 20 seconds)
Status Job ID Name Coverage
  Flatpak
passed #258118001
aarch64
app_aarch64

01:52:28

passed #258118006
armhf
app_arm

00:43:44

passed #258117995
cache_i686 x86_64
app_i686

01:27:34

passed #258117987
cache_x86_64 x86_64
app_x86_64

01:06:49

 
  Vm
manual #258118026
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64
passed #258118035
armhf
minimal_systemd_vm_arm

00:07:32

passed #258118019
cache_i686 x86_64
minimal_systemd_vm_i686

00:51:06

passed #258118011
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

00:36:35