Document how the shim expects to have information configured on a container

[kate-goldenring]

Sometimes SpinKube users may prefer to skip using the Spin Operator and choose to define their own K8s deployments, for example if using Argo Rollout resources. We should document how the Shim expects information such as OTEL parameters, etc to be passed inside PodSpecs (usually as Container environment variables) to make it easier for users to construct their own deployments.