Let's update the CentOS Installation Guide to reflect the current
information on how to install kata packages provided by the
Virtualiation Special Interest Group.
These are not official CentOS packages, as those are not coming from Red
Hat Enterprise Linux. These are the same packages we have on Fedora and
we have decided to keep them up-to-date and sync'ed on both Fedora and
CentOS, so people can give Kata Containers a try also on CentOS.
The nature of these packages makes me think that those are "as official
as they can be", so that's the reason I've decided to add the
instructions to the "official" table.
Together with the change in the Installation Guide, let's also update
the README and reflect the fact we **strongly recommend** using CentOS
8, with the packages provided by the Virtualization Special Interest
Group, instead of using the CentOS 7 with packages built on OBS.
Fixes: #623
Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>