Add cli guide for nebula

[jasikpark]

❯ 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

[jasikpark]

Mentioned in slackhq/nebula#53