Add section about scalability in readme.md and create some doc.

[MalloZup]

Would be nice to have in the doc stated:

• due to the nature of libvirt- multiple URI we can use different KVM server witch multiples the combination of resources.
• A note about how we manage the saltstack minions, we create them dinamically via roster

Consider other aspect of scalability: we can add DISK for ceph by demand

A diagram/chart would be wonderfull to have ( pictures are more then words)