Expose preferred_syntax configuration in README

This is the most important configuration one can set for sass-rails, yet the only reliable place to find it is [StackOverflow](http://stackoverflow.com/questions/6006360/how-to-make-rails-3-1-use-sass-over-scss-as-the-default).
Extract preferred_syntax from the list of Sass option caveats

Also switched to unordered lists since the order had no meaning and did not 
correspond to any Sass option order.
Merge pull request #211 from olivierlacan/master

Expose preferred_syntax configuration in README