Update release schedule, notes

Bug: skia:
Change-Id: I993eec22654edf93ea93e37282b0e08d19acd589
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/281516
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Heather Miller <hcm@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index ddde299..53c3640 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -4,6 +4,12 @@
 
 * * *
 
+Milestone 84
+
+  * <insert new release note here>
+
+* * *
+
 Milestone 83
 
   * Remove localmatrix option from SkShaders::[Blend, Lerp]