i18n: mention "TRANSLATORS:" marker in Documentation/CodingGuidelines

These comments have to have "TRANSLATORS: " at the very beginning
and have to deviate from the usual multi-line comment formatting
convention.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -164,6 +164,16 @@ For C programs:
 	 * multi-line comment.
 	 */
 
+   Note however that a comment that explains a translatable string to
+   translators uses a convention of starting with a magic token
+   "TRANSLATORS: " immediately after the opening delimiter, even when
+   it spans multiple lines.  We do not add an asterisk at the beginning
+   of each line, either.  E.g.
+
+	/* TRANSLATORS: here is a comment that explains the string
+	   to be translated, that follows immediately after it */
+	_("Here is a translatable string explained by the above.");
+
  - Double negation is often harder to understand than no negation
    at all.