This warning is triggered if a pull request adds a file named new-file.txt (or similar).
Such a filename is indicative of pull requests which were created to test out Git or GitHub, rather than proposing any legitimate change to Bootstrap.
If you want to test out Git or GitHub, create your own personal repository and conduct your experiments there instead, where they won't bother anyone. Using the repositories of public projects (such as Bootstrap) for such experiments wastes the time of those projects' maintainers and is thus considered rude & annoying. Which is why your pull request was automatically closed.
To learn more about using Git or GitHub, check out the links on the following GitHub Help page: https://help.github.com/categories/bootcamp/