Introducing draft pull requests

At GitHub, we’ve always felt that you should be able to open a pull request to start a conversation with your collaborators as soon as your brilliant idea or code is ready to take shape. Change the status to “Ready for review” near the bottom of your pull request to remove the draft state and allow merging according to your project’s settings. Draft pull requests are ready for your code in public and open source repositories, as well as in private repositories for groups using GitHub Team and Enterprise Cloud.

Source: github.blog