docs: document requirement to provide Signed-off-by lines for DCO

docs: document requirement to provide Signed-off-by lines for DCO

Document that contributors are required to assert compliance with the
Developers Certification of Origin 1.1, by providing Signed-off-by tags
for all commit messages. The DCO is formally stating what we have long
implicitly expected of contributors in terms of their legal rights to
make the contribution. This puts the project in a stronger position
should any questions around contributions be raised going forward in the
future.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>