[java-atk-wrapper] HACKING: mention global references

[Samuel Thibault <sthibaul src gnome org>]

commit 7810745300acc7bbf4bd98a09976ad930567b5b8
Author: Samuel Thibault <samuel thibault ens-lyon org>
Date:   Fri Aug 2 19:15:52 2019 +0200

    HACKING: mention global references

 HACKING | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/HACKING b/HACKING
index b0846c9..c7e89ca 100644
--- a/HACKING
+++ b/HACKING
@@ -136,3 +136,8 @@ The jint object is mapped to C's long which is always at least 32 bits), instead
 of of C's int (which could be 16 bits). The resource for glib types can be
 found at <https://developer.gnome.org/glib/stable/glib-Basic-Types.html>
 
+JAW_GET_* acquire a global reference on the obtained jobject (because local
+references are dropped when ), it needs to be released before returning in all
+code paths!
+
+TODO: probably we can safely use mere (non-weak) local references here?