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

[htgoebel]

The README does not state anything about towncrier.check. NEWS.rst 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?

[achimnol]

I just found that towncrier.check only checks the existence of news fragments, not their numberings with the current PR number or other IDs. It would be nice to have a configuration to specify the environment variable name and/or pattern to extract and compare the IDs from the file names of fragments with the ID from the CI environment or an optional cli argument.

[altendky]

@achimnol, thanks for the suggestion. That sounds like a feature request deserving of its own issue in addition to the mention here? It sort of plays into the discussion in #309 as well, so reference that, but at the moment I think a dedicated ticket would be good.