Brief description of options in README

Hi,

Would be super helpful to have some brief description of the options in the README. I didn't realise i could just use `{:background true}` to background the process instead of a messy `(sh "bash" "-c" "blah &")` type function, until i stumbled across it browsing the code.

Kind regards,

Brett

p.s. thanks for the library, super clean way to call shell processes.