Add cli guide for `nebula`

```
❯ nebula -h
Usage of nebula:
  -config string
    	Path to either a file or directory to load configuration from
  -help
    	Print command line usage
  -service string
    	Control the system service.
  -test
    	Test the config and print the end result. Non zero exit indicates a faulty config
  -version
    	Print version
```