Add 'cpu_threshold' to documentation

Hi @gyptazy, (Not a bug, more a reminder). Empirically I found that the setting `cpu_threshold` exists, just like `memory_threshold`. It is missing in the readme.md and proxlb_example.yaml Of course it could be a undocumented feature and you like it to stay that way :-). (The official docs are missing the list of options/settings for the config file alltogehther.) Thanks, Frank