This commit includes a light-weight approach for MetalLB to adopt the
DCO (Developer Certificate of Origin) for contributions.  Some projects
use a `Signed-off-by:` header in commit messages as an acknowledgement
of the DCO.  In this case, we just include the DCO file and update the
contributing docs to explain that by contributing, you agree to the DCO.

Some examples of other projects that adopted this approach to using the
DCO include:

 - several from github.com/coreos, such as github.com/coreos/ignition
 - projects under github.com/metal3-io/