Refactor README.md file, add new install option, add version flag

[mszostok]

Description

• Add functionality to print binary version via --version flag

  
  # Print full version
  $ codeowners-validator -v
  version.Info{Version:"0.1.1", GitCommit:"b0e711f", BuildDate:"2019-11-13T08:15:18Z", GoVersion:"go1.13.3", Compiler:"gc", Platform:"darwin/amd64"}

Print just the version number

$ codeowners-validator -v --short Version: 0.1.1

- Add new install option via script, refactor README.md file