Create new Separate menu for custom module in admin config page in drupal 8

Adding menu in admin config page under services , content or development is simple as we need to create a file in our custom module root path named modulename.links.menu.yml

modulename.settings:
  title: ' settings'
  description: ' selection.'
  route_name: modulename.settings
  parent: 'system.admin_config_development'
modulename.second_link:
  title: 'settings'
  description: 'selection.'
  route_name: modulename.settinganother
  parent: 'modulename.settings'  
  menu_name: modulename

what if you want create new separate menu like  development, services, content in admin config for your custom module

Simple  trick

Create same file with small alteration  modulename.links.menu.yml

look at the highlighted line.

modulename.admin_config_modulename:
  title: ' settings'
  description: ' selection.'
  route_name: modulename.settings
  parent: 'system.admin_config'
modulename.second_link:
  title: 'settings'
  description: 'selection.'
  route_name: modulename.settinganother
  parent: 'modulename.admin_config_modulename'  
  menu_name: modulename

Done