update release notes

Change-Id: If6157c345b0329665ac385de23e42d4cd7331cbc
Docs-Preview: https://skia.org/?cl=376847
TBR Joe
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/376847
Reviewed-by: Heather Miller <hcm@google.com>
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Heather Miller <hcm@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index 71b2b5a..a99c73f 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -4,6 +4,13 @@
 
 * * *
 
+Milestone 91
+------------
+
+ *
+
+* * *
+
 Milestone 90
 ------------
   * Renamed use of sk_cf_obj in external Metal types to sk_cfp.