New feature, improvement proposal

Add a GitHub ribbon by default on the right-hand side like currently used on https://maven.apache.org/skins/maven-fluido-skin/ to encorage forking/ease finding the source code.

The configuration is described in https://maven.apache.org/skins/maven-fluido-skin/#GitHub_Ribbons and should use the {project.artifactId} property for referencing the GitHub project below the apache organization.