Fix README.rdoc with updated libcroco installation instructions

Created by: jcanfield

Version 0.6.4 of libcroco contained a bug:

which is essential to css_doc: it does not parse comments, but instead it simply ignores them..

The latest version of libcroco which is 0.6.8 has fixed that bug. By using Homebrew instead of Mac Ports and editing the Homebrew Formula you are able to edit the URL and SHA256 lines and download and install a proper libcroco.

I've found various forks of this project but nobody has updated their install instructions which lead to a good 20-30 minutes of tinkering with it before I was able to use cssdoc on OSX.

I have also submitted this pull request to the rdougan fork of css-doc.

Signed-off-by: Joshua Canfield jcanfield@creativeboulder.com