'A configurator directive which registers a free-standing
permission without associating it with a view callable.  This can be
used so that the permission shows up in the introspectable data under
the ``permissions`` category (permissions mentioned via ``add_view``
already end up in there).  For example::
config = Configurator()
config.add_permission(\'view\')'

def add_permission(self, permission_name):

    intr = self.introspectable('permissions', permission_name, permission_name, 'permission')
    intr['value'] = permission_name
    self.action(None, introspectables=(intr,))
