docs: Provide an article on how to add a custom runner to the project

docs: Provide an article on how to add a custom runner to the project

Since running our functional test suite in GitLab cannot make use of
the shared resources it makes sense to document the process of adding
own HW to run the custom libvirt executor that powers the integration
suite. This article will likely make even more sense in the future with
GitLab severely cutting down on shared CI resources.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>