Returns the script's main help text, as a string.

def print_help_info(self, global_options):
        """
        Returns the script's main help text, as a string.
        """
        usage = ['',"Type '%s help <subcommand>' for help on a specific subcommand." % self.prog_name,'']
        usage.append('Available subcommands:')
        commands = list(self.get_commands().keys())
        commands.sort()
        for cmd in commands:
            usage.append('  %s' % cmd)
        return '\n'.join(usage)