Please document what `towncrier.check` check for and how to use it

The README does not state anything about `towncrier.check`. [NEWS.rst](https://github.com/hawkowl/towncrier/blob/master/NEWS.rst#towncrier-1860-2018-07-05) says:
> Features:
> * `python -m towncrier.check`, which will check a Git branch for the presence of added newsfiles […]

Please add a few lines of documentation:
What exactly does `towncrier.check` check for? What is its intended usage?