This is done using GitHub Issues.
Use issues to track ideas, enhancements, tasks, or bugs for work on GitHub.
For more information on issues see:
This is done using GitHub Pull Requests.
Pull requests let you tell others about changes you've pushed to a repository on GitHub. Once a pull request is opened, you can discuss and review the potential changes with collaborators and add follow-up commits before the changes are merged into the repository.
For more information on Pull Requests see:
When opening a pull request in a Particular Software repository, the CLA assistant will verify that the GitHub account submitting the pull request has signed the Particular Software Contributor License Agreement. Pull requests cannot be accepted until the CLA has been signed.