add README file

Obviously most of the work for this was done by @joliss, so she gets the
credit.

I think the main part missing from this is how to use the options that
were added to ease interaction with ruby, such as bundleExec, loadPath,
and require
Replace quickTemp with broccoli-writer

Inspired by this change:
https://github.com/joliss/broccoli-sass/commit/a1c305f473e631af1b60ef4925f8e55d25910e15