add README
    
    Import the content from the fedorahosted.org Trac wiki's homepage and
    convert it to markdown.
    
    This provides an introduction to Koji for users who are browsing the
    Pagure UI.
    
    fedorahosted.org is going away:
    https://lists.fedoraproject.org/archives/list/announce@lists.fedoraproject.org/message/RLL3LFUPLYMAUKGZ5B3O64XKJXBT24KZ/