README.md: Update "yk help" output to match reality
README.md: Improve the "Usage" section

  * Remove redundant sections from the intro text, move everything into
    one place.

  * Change terminology: s/subcommand/command/

  * Use four spaces instead of ``` to delimit "yk help" output. It is
    more readable in plaintext.

  * Other small improvements.
README.md: Remove examples of executed commands

I don't think they are very helpful as they all look more or less the
same. They also create maintenance burden as they need to be updated
when command output changes.
Merge pull request #389 from yast/readme-usage-improvements

[Review] Request from '' @ 'yast/ycp-killer/readme-usage-improvements'