CodingGuidelines: instruct to name arrays in singular

CodingGuidelines: instruct to name arrays in singular

Arrays should be named in the singular form, ensuring that when
accessing an element within an array (e.g. dog[0]) it's clear that
we're referring to an element instead of a collection.

Add a new rule to CodingGuidelines asking for arrays to be named in
singular instead of plural.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>