Fix README.rdoc with updated libcroco installation instructions

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](https://github.com/rdougan/css_doc) fork of ```css-doc```.

Signed-off-by: Joshua Canfield jcanfield@creativeboulder.com