Add documentation to indicate that GitLab CI Multi Runner can read `KUBECONFIG`
We should add documentation to indicate that gitlab-ci-multi-runner
can read the KUBECONFIG
data and utilize the data contained. This means it can support bearer auth, for example.
This just means that a machine needs to populate that data, rather than require specific configuration in config.toml
.
Discussion can be found here: https://gitlab.com/gitlab-org/gitlab-ce/issues/28561#note_24054767