improvement: clarify SUID/SGID options in readme

make sure that remove_from_unkown and dry_run_on_unkown are better understandable in the readme.

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
Merge pull request #16 from TelekomLabs/readme_sugid

improvement: clarify SUID/SGID options in readme