better usage in readme and --help, maybe a manpage

[Baerbeisser]

Assuming i'm not a user of linguist tool, only way to figure out usage i found is --help

  enry v2.1.0 build: 08-07-2019_17_39_18 commit: 3c3ea9a, based on linguist commit: 3a1bd3c
  enry, A simple (and faster) implementation of github/linguist
  usage: enry [-mode=(file|line|byte)] [-prog] <path>
         enry [-mode=(file|line|byte)] [-prog] [-json] [-breakdown] <path>
         enry [-mode=(file|line|byte)] [-prog] [-json] [-breakdown]
         enry [-version]

which is good enough for a short overview, which --help is for. But things missing are:

• -all option mentioned in README.md
• Explanation of each option (what it is for)
• usage of each option, such as
  ** explanation of file|line|byte
  ** do --json/--breakdown mentioned in README.md need a filename/full path to write to?
  ** used files/environment variables

btw, is it --json/--breakdown as in README.md or -json/-breakdown as reported by --help? And for the other options, - or --?

I mean, i know writing documentation can be a pain, but this is a bit too meager.