CodingGuidelines: describe "export VAR=VAL" rule

https://lore.kernel.org/git/201307081121.22769.tboegi@web.de/
resulted in 9968ffff (test-lint: detect 'export FOO=bar',
2013-07-08) to add a rule to t/check-non-portable-shell.pl script to
reject

export VAR=VAL

and suggest us to instead write it as two statements, i.e.,

VAR=VAL
export VAR

This however was not spelled out in the CodingGuidelines document.

We may want to re-evaluate the rule since it is from ages ago, but
for now, let's make the written rule and what the automation
enforces consistent.

Signed-off-by: Junio C Hamano <gitster@pobox.com>