Add tune=ssimulacra2 improvements to the CHANGELOG

Bug: aomedia:401234702
Change-Id: If219e62e138b61988f3b22510716e6ded390147f

CHANGELOG

diff --git a/CHANGELOG b/CHANGELOG
index 99d128e..445d744 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,8 +1,13 @@
 yyyy-mm-dd next version
   This release is ABI compatible with the last release.
 
+  - Compression Efficiency Improvements
+    * Some minor quantization matrix formula and delta chroma q tweaks for the
+      AOM_TUNE_SSIMULACRA2 tuning mode, with up to 0.2% SSIMULACRA 2 score
+      efficiency gains.
+
   - Speedups
-    * Make Adaptive CDEF more decoder-friendly by limiting CDEF application to
+    * Make Adaptive CDEF more decoder friendly by limiting CDEF application to
       only regions where high filter strengths are needed.
 
 2025-09-05 v3.13.1