Add .gitmodules for submodule kube-prometheus

Please add a .gitmodules files to the repository that clarifies where the submodule kube-prometheus can be downloaded. Otherwise cloning the repository with --recursive will fail.

In addition, I wasn't able to find the specific commit currently referenced in the repository: https://github.com/prometheus-operator/kube-prometheus/commit/4c71178fae6ee81e19ced15324ed3b3a5e0d250d