[devhelp] HACKING file: explain why not using the G_DECLARE macros

[Sébastien Wilmet <swilmet src gnome org>]

commit 9d21f44418bdfb6243b5c8986cab4aa2495e1b2b
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sun May 27 07:40:19 2018 +0200

    HACKING file: explain why not using the G_DECLARE macros

 HACKING | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/HACKING b/HACKING
index cd995ea8..fc16ff23 100644
--- a/HACKING
+++ b/HACKING
@@ -21,5 +21,9 @@ Devhelp follows the Linux kernel coding style, with some exceptions:
          * Second line.
          */
 
+GObject classes must not be created with a G_DECLARE macro, it causes more
+problems than it solves:
+https://blogs.gnome.org/swilmet/2015/10/10/changing-quickly-between-a-final-and-derivable-gobject-class/
+
 See also:
 https://developer.gnome.org/programming-guidelines/unstable/