Modify docs to add more clarity to exclusion list and kube-system

[erewok]

Hello,

Thanks for working on this project! It's been really helpful for managing our clusters.

Based on our conversation in #122, it seemed like it might help if we included a little more detail around the exclusion list and whether or not kube-system is excluded by default. Let me know if this works or if you prefer something different.

Closes #122

[coveralls]

Coverage remained the same at 94.064% when pulling cba94e2 on erewok:exclude_kube_sys_doc into 14720bd on hjacobs:master.