Policy on bugbears in usage of popular third party libraries

Hi, currently flake8-bugbear seems to primarily (only?) contain errors/warnings either inherent to python code or constructs from the python stdlib, but there are some popular third party libraries, such as MarkupSafe which also have very common usage mistakes, that are almost certainly bugs.

For MarkupSafe there is an existing flake8-plugin, however it hasn't been updated in three years and isn't published on pypi, so I'd feel a little bit better if it was part of a larger, actively maintained project.

So for now I'd just like to get a feel what the maintainers stance on this is, and if detecting such errors are welcome additions, I would be happy to submit a PR for this MarkupSafe specific issue to begin with. It would also be nice to have a contribution policy alongside the development instructions, so it's easier to gauge if it's worth opening a pull request for new errors/warnings or it'd be better suited as a standalone plugin.