Add details on how to assign Runner Manager to security fork project
What does this MR do?
Adds missing details in the process of registering the new Windows Runner Managers.
Why was this MR needed?
To have a more complete step-by-step guide.
What's the best way to test this MR?
What are the relevant issue numbers?
https://ops.gitlab.net/gitlab-com/gl-infra/ci-infrastructure-windows/-/merge_requests/92