Add information about Graphviz dependency on README
Add header on README to separate sections on Dependencies
Change use of direct: :true to group: :direct on Gemfile
Merge pull request #3 from fernandes/change_from_direct_to_group

Until some time ago this year, the extraneous option `direct:` worked, but then a check was added to not allow extra options and the feature has been broken ever since.

I love your way around it. Perfect hack! Merging it in.