[ekiga] Add info about callback in HACKING file

[Eugen Dedu <ededu src gnome org>]

commit dba363ece24002a8191e547a6481a6ca8e9abf38
Author: Eugen Dedu <Eugen Dedu pu-pm univ-fcomte fr>
Date:   Sat Dec 18 11:36:31 2010 +0100

    Add info about callback in HACKING file

 HACKING |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/HACKING b/HACKING
index d99173d..b3ff7e2 100644
--- a/HACKING
+++ b/HACKING
@@ -26,4 +26,6 @@ Developers:
 - Encoding: Ekiga is completely UTF-8.  The only exception is that the
   (audio/video) device names are in latin-1 for Linux <= 2.6.29, and
   system codepage for Windows.  The latin2utf and utf2latin functions
-  take care of that.
\ No newline at end of file
+  take care of that.
+- To find out which callbacks are called when o value in gconf
+  changes, search gm_conf_notifier_add in ekiga code.