added homebrew-specific header path fix to readme

I went to use this in a mac with homebrew and got include and linker errors on compile. I traced it down to xcode messing with the default include and linker paths and removing the default gcc behavior of looking in `/usr/lib/...` (which is where homebrew puts the headers and libs). Just wanted to update the docs and save others the work I went through.

Do you want me to submit another pull request to gh-pages with the same verbiage?