[easytag] Mention use of C89 comment style in HACKING

[David King <davidk src gnome org>]

commit 2437156f61115e1995c1ecc07133acf1d37c66a6
Author: David King <amigadave amigadave com>
Date:   Wed Mar 2 08:13:41 2016 +0000

    Mention use of C89 comment style in HACKING

 HACKING |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/HACKING b/HACKING
index 2d0621c..4638651 100644
--- a/HACKING
+++ b/HACKING
@@ -29,6 +29,7 @@ The original EasyTAG coding style was close to K&R style, with four-space
 indents. This has been changed slightly to the following guidelines:
 
 * BSD/Allman brace style
+* C89 comments
 * one space before opening bracket, none after opening or closing
 * four space indent
 * try to keep to 80 character lines