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>

Documentation/CodingGuidelines

@@ -499,6 +499,33 @@ For Python scripts:
  - Where required libraries do not restrict us to Python 2, we try to
    also be compatible with Python 3.1 and later.
 
+
+Program Output
+
+ We make a distinction between a Git command's primary output and
+ output which is merely chatty feedback (for instance, status
+ messages, running transcript, or progress display), as well as error
+ messages. Roughly speaking, a Git command's primary output is that
+ which one might want to capture to a file or send down a pipe; its
+ chatty output should not interfere with these use-cases.
+
+ As such, primary output should be sent to the standard output stream
+ (stdout), and chatty output should be sent to the standard error
+ stream (stderr). Examples of commands which produce primary output
+ include `git log`, `git show`, and `git branch --list` which generate
+ output on the stdout stream.
+
+ Not all Git commands have primary output; this is often true of
+ commands whose main function is to perform an action. Some action
+ commands are silent, whereas others are chatty. An example of a
+ chatty action commands is `git clone` with its "Cloning into
+ '<path>'..." and "Checking connectivity..." status messages which it
+ sends to the stderr stream.
+
+ Error messages from Git commands should always be sent to the stderr
+ stream.
+
+
 Error Messages
 
  - Do not end error messages with a full stop.