Fixed #23860 -- Document imports order convention.

Please see [#23860](https://code.djangoproject.com/ticket/23860).
Seems the blank line is used more often that not, 14 files without it versus 196 with it (at least as determined by ``grep -RPzl "from __future__ import \w+\n\nimport \w+" --include=*.py . | wc -l``). Added.