CodingGuidelines: document which output goes to stdout vs. stderr

CodingGuidelines: document which output goes to stdout vs. stderr

It has long been practice on this project for a command to emit its
primary output to stdout so that it can be captured to a file or sent
down a pipe, and to emit "chatty" messages (such as those reporting
progress) to stderr so that they don't interfere with the primary
output. However, this practice is not necessarily universal; another
common practice is to send only error messages to stderr, and all other
messages to stdout. Therefore, help newcomers out by documenting how
stdout and stderr are used on this project.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>