CodingGuidelines: document NEEDSWORK comments

CodingGuidelines: document NEEDSWORK comments

We often say things like /* NEEDSWORK: further _do_ _this_ */ in
comments, but it is a short-hand to say "We might later want to do
this.  We might not.  We do not have to decide it right now at this
moment in the commit this comment was added.  If somebody is
inclined to work in this area further, the first thing they need to
do is to figure out if it truly makes sense to do so, before blindly
doing it."

This seems to have never been documented.  Do so now.

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