Update README.rdoc

Update Singleton section of Readme, to accord undocumented yet changes (https://github.com/josevalim/inherited_resources/commit/cfb2d88c6d93dbaf9bd4a81996a660181fd2b2e1 - Allow for nesting of singleton belong_to's.)
Merge pull request #269 from iantropov/master

Update Singleton section of Readme