Mention "local convention" rule in the CodingGuidelines

The document suggests to imitate the existing code, but didn't
say which existing code it should imitate. This clarifies.

Signed-off-by: しらいしななこ <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -21,8 +21,13 @@ code.  For git in general, three rough rules are:
 
 As for more concrete guidelines, just imitate the existing code
 (this is a good guideline, no matter which project you are
-contributing to).  But if you must have a list of rules,
-here they are.
+contributing to). It is always preferable to match the _local_
+convention. New code added to git suite is expected to match
+the overall style of existing code. Modifications to existing
+code is expected to match the style the surrounding code already
+uses (even if it doesn't match the overall style of existing code).
+
+But if you must have a list of rules, here they are.
 
 For shell scripts specifically (not exhaustive):