CodingGuidelines: describe naming rules for configuration variables

CodingGuidelines: describe naming rules for configuration variables

We may want to say something about command line option names in the
new section as well, but for now, let's make sure everybody is clear
on how to structure and name their configuration variables.

The text for the rules are partly taken from the log message of
Jonathan's 6b3020a2 (add: introduce add.ignoreerrors synonym for
add.ignore-errors, 2010-12-01).

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