Add a way to show a contributing information block when opening a new PR
    
    When someone opens a new PR, we'll look into the requests git repo for
    a file ``templates/contributing.md`` and if present, we will show it on
    the new PR page to the user.
    
    This will allow developers to express what they would like to see in a
    PR when submitted.
    
    fixes https://pagure.io/pagure/issue/587