[retro-gtk] hacking: Document usage of G_GNUC_WARN_UNUSED_RESULT

[Adrien Plazas <aplazas src gnome org>]

commit fe81e515776cafb5c67e8729eac62b29112b79d6
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Fri Apr 16 22:30:00 2021 +0200

    hacking: Document usage of G_GNUC_WARN_UNUSED_RESULT

 HACKING.md | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/HACKING.md b/HACKING.md
index 21abcba..6b9dde8 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -87,6 +87,11 @@ function (…) {
 }
 ```
 
+### Function Result
+
+If a public function transfers ownership of its returned value, explicit that by
+giving the function the `G_GNUC_WARN_UNUSED_RESULT` attribute.
+
 ### Function Order
 
 Functions are defined in the following order: