The instructions below show how to create a menu link to the custom_module module in the Development section of the Admin>>Configuration page (http:///admin/config).
In the root folder of your module, create a new file with the name custom_module.links.menu.yml, and add the following code
custom_module.admin:
title: 'Custom Module Settings'
description: 'example of how to make an admin settings page link'
parent: system.admin_config_development
route_name: custom_module.content
weight: 100