A CONTRIBUTING file in the root directory of open-source projects serves as a guideline for contributors on how to format code, test fixes, and submit patches, thereby facilitating effective collaboration between maintainers and contributors. By adding such a file, maintainers can clearly communicate their preferred contribution policies, allowing contributors to easily verify that their submissions align with the project's guidelines. The text encourages maintainers to include a CONTRIBUTING file (or CONTRIBUTING.md for Markdown users) in their repositories, which will then be linked whenever a contributor creates an Issue or opens a Pull Request. If a project lacks this file, contributors are encouraged to initiate a pull request to add one, ensuring that collaboration remains straightforward and organized.