Use README.adoc as explicit doc URL
Use README.adoc as explicit doc URL

Implicit doc URL was failing for git client

MarkEWaite/git-plugin,martinda/git-plugin,MarkEWaite/git-plugin,jenkinsci/git-plugin,jenkinsci/git-plugin,jenkinsci/git-plugin,MarkEWaite/git-plugin,MarkEWaite/git-plugin,martinda/git-plugin,martinda/git-plugin,jenkinsci/git-plugin