Note aom_roi_map_t change is considered compatible

Explain in the changelog for the next release why the change to the
aom_roi_map_t struct does not break ABI compatibility.

Change-Id: I86e51ccd6b0e0bda51283a51e33c713b47ca0df1

CHANGELOG

diff --git a/CHANGELOG b/CHANGELOG
index 6d0cb69..34ea55f 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,4 +1,11 @@
 2025-mm-dd next version
+  This release is ABI compatible with the last release.
+
+  The aom_roi_map_t struct, used only by the codec control
+  AOME_SET_ROI_MAP, was modified in this release. Since AOME_SET_ROI_MAP
+  was unimplemented, aom_roi_map_t was effectively an unused struct.
+  Therefore aom_roi_map_t is to be considered as a new struct added in
+  this release and this change does not break ABI compatibility.
 
 2025-04-11 v3.12.1
   This release includes several bug fixes. This release is ABI