Brief description of options in README

[brettbates]

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.