README: Add intro about the configuration file

The 'Guarding unsafe tests' section mention the unittests.cfg
file which was never introduced before. In this change
it was added a section with a few words about the tests
configuration file (unittests.cfg).

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>