CONTRIBUTING: Include information on build dependencies

CONTRIBUTING: Include information on build dependencies

libvirt depends on a ton of packages, so trying to install them
all by using the classic approach of repeatedly running configure
and reacting to each failure by installing the corresponding
missing package will inevitably lead to frustration.

Luckily there's an easy solution to get most dependencies
installed in one fell swoop, and we just need to document it.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>