README.md: Improve the "Usage" section
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.

yast/ycp-killer,yast/ycp-killer