Document the required JupyterHub permission to run the culler given passed flags

[consideRatio]

The permissions we need to request for idle-culler depends on what it is started to do. If it needs to cull users, it means it needs permissions to delete users. If it is to stop named servers, perhaps it needs some specific permission for named servers as well?

This issue represent the need to document the base permissions needed and the additional permissions needed depending what flags are passed to idle culler during startup.