Add release note on Metal resource tracking change

Bug: skia:12253
Change-Id: Iaf7cb2f2f3ee9310cc5edc8425d7123d11857055
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/438537
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index b17bf01..26f2035 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -6,6 +6,10 @@
 
 Milestone 94
 ------------
+  * Metal backend has been changed to track command buffer resources manually
+    rather than using retained resources.
+    https://review.skia.org/432878
+
   * Added virtual onResetClip() to SkCanvas for Android Framework, to emulate the soon-to-be-removed
     expanding clip ops guarded by SK_SUPPORT_DEPRECATED_CLIPOPS.
     https://review.skia.org/430897