update release notes for M91 branch

Change-Id: I5bddb5aa3543953cf5fb6090a247359ab69a239c
Docs-Preview: https://skia.org/docs/user/release/release_notes?cl=395097
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/395097
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Heather Miller <hcm@google.com>
Auto-Submit: Heather Miller <hcm@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index 07e7348..bfeef83 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -4,6 +4,12 @@
 
 * * *
 
+Milestone 92
+------------
+  *
+
+* * *
+
 Milestone 91
 ------------
   * The SkSL DSL API has been moved into public headers, although it is still under active