need to provide guidance on sandbox overhead

[egernst]

orchestrators should expect a pod created by Kata Containers to incur a non-negligible overhead on top of the container resource requests.

In Kubernetes, work is being done to formally register and apply this overhead. See PodOverhead KEP.

Related to the outstanding PR #428, we should introduce the concept and provide guidance for memory and CPU overhead of running a pod / container with Kata.

[gkhanna79]

CC @gkhanna79