README.md: Add clarification note about project scope

Related with #104
10 jobs for jjardon/scope_clarification in 143 minutes and 31 seconds (queued for 16 seconds)
Status Job ID Name Coverage
  Flatpak
passed #258095575
aarch64
app_aarch64

01:40:52

passed #258095578
armhf
app_arm

00:35:28

passed #258095567
cache_i686 x86_64
app_i686

01:16:10

passed #258095582
ppc64le allowed to fail
app_powerpc64le

01:02:37

passed #258095560
cache_x86_64 x86_64
app_x86_64

01:12:50

 
  Vm
manual #258095602
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64
passed #258095605
armhf
minimal_systemd_vm_arm

00:08:01

passed #258095589
cache_i686 x86_64
minimal_systemd_vm_i686

00:42:33

manual #258095607
ppc64le allowed to fail manual
minimal_systemd_vm_powerpc64le
passed #258095586
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

00:39:59