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

>  I thought we had previously agreed that PEP8 + line lengths up to 100 was a style to adhere to as far as possible.

In practice we do have stylistic differences between subpackages (`io.fits` is basically 80, `table`, `time` are basically 100).  Also in practice each subpackage has 1-3 "moderators" that do final review and ensure code quality and style, so that naturally maintains the subpackage style distinctions.

I don't mind that much if new Table code is stopped at 80 characters (though in my personal view that tends to make code less readable), but I do mind when people reformat existing code purely as a stylistic change (normally unrelated to the actual content of the PR).  I also suppose that @embray might prefer if new code in io.fits stays within 80 characters.