[gtksourceview/wip/chergert/gsv-gtk4] doc: update HACKING for ownership transfer

[Christian Hergert <chergert src gnome org>]

commit 7035d3ed2ec40beafd23863c6929c8ebd437a58c
Author: Christian Hergert <chergert redhat com>
Date:   Fri Jan 17 13:40:05 2020 -0800

    doc: update HACKING for ownership transfer

 HACKING | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/HACKING b/HACKING
index 237793de..f499c922 100644
--- a/HACKING
+++ b/HACKING
@@ -142,6 +142,9 @@ Here are some general advices:
     validate invariants. These are used in debug builds but can be compiled
     out of production/release builds.
 
+  - When transfering ownership of an object or struct, use g_steal_pointer()
+    to make it clear when reading that ownership was transfered.
+
 See also:
 https://blogs.gnome.org/swilmet/2012/08/01/about-code-quality-and-maintainability/