add install instructions for osx

There are other ways to do this, but this seems to be the most commonly
used and easiest way to get the job done. Alternate methods could be
added but at least having this one will alert folks that using this on
OSX is an option.
use more portable find syntax

BSD find (e.g. as found on Mac OSX) has more strict rules for parsing
arguments and needs a path. PWD is not assumed as on GNU find.