Clarify CONTRIBUTING file
Clarify CONTRIBUTING file

martinda/git-plugin,v1v/git-plugin,jenkinsci/git-plugin,MarkEWaite/git-plugin,v1v/git-plugin,jacob-keller/git-plugin,jenkinsci/git-plugin,jenkinsci/git-plugin,MarkEWaite/git-plugin,jacob-keller/git-plugin,jenkinsci/git-plugin,v1v/git-plugin,martinda/git-plugin,MarkEWaite/git-plugin,MarkEWaite/git-plugin,jacob-keller/git-plugin,martinda/git-plugin