update SELinux README
    
    The documentation for SELinux has grown a little stale.  Refresh it and
    convert it to markdown syntax¹.
    
    Remove outdated data about the graphical system-config-selinux tool.
    
    Mention that restorecon might be needed to update /var/lib/git.
    
    Use the semanage equality option (-e) to simplify the command used to
    add an alternate location for git repositories.
    
    ¹ The main reason to convert to markdown is to avoid pagure displaying
      it as one large blob.