document command line flags and system env vars

[filex]

couper -h
Usage of couper:
  -f string
    	-f ./couper.conf (default "couper.hcl")
  -log-format string
    	-log-format json (default "default")
  -p int
    	-p 8080 (default 8080)
  -xfh
    	-xfh

…

[malud]

Since this is solved with #187 couper returns

go run main.go -h

Couper usage:
  couper <cmd> <options>

Available commands:
  run		Start the server with given configuration file.
  version	Print the current version and build information.
  help		Usage for given command.

Examples:
  couper run
  couper run -f couper.hcl
  couper run -watch -log-format json -log-pretty -p 3000

Usage of global options:
  -f string
    	-f ./my-path/couper.hcl (default "couper.hcl")
  -log-format string
    	-log-format=json (default "common")
  -log-pretty
    	-log-pretty
  -watch
    	-watch
  -watch-retries int
    	-watch-retries 10 (default 5)
  -watch-retry-delay duration
    	-watch-retry-delay 1s (default 500ms)

I will close this one.