Advantages for maintainers
If you are a gem developer or maintain a library you probably already use badges in your README to show that your tests are passing and your code isn't a horrible mess in need of refactoring.
Now you can also show that you documented your project properly.
Advantages for developers
If you are a Ruby developer, you know the pain of cloning a project you want to improve just to find out that it is completely undocumented.
Then you have to form the mental equivalent of an AST to comprehend how it all fits together. Wouldn't it be great to see the present level of documentation directly in the README?
Further reading: http://inch-pages.github.io