Add a point to coding style docs to follow existing subpackage style

I'm not sure if I'd mention 'the preference of the primary developers' because it makes it seem like we don't allow community input. I don't think it's good to make it sound like we have different styles in different sub-pakages, and I think it'd be better if we can agree now what our guidelines for style are - in particular I thought we had previously agreed that PEP8 + line lengths up to 100 was a style to adhere to as far as possible (it was a couple of years back), and there wasn't any disagreement about that?

Large stylistic pull requests have to be planned to not conflict with other PRs, so that's a better argument for now allowing random PRs from users to just fix style - it has to be coordinated better.