add M86 branch release notes

Bug: skia:
Change-Id: I62331cab6cfadbaf9ab2f74e316cb7a2515249d0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/312305
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 4ebaa87..410c60e 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -4,6 +4,13 @@
 
 * * *
 
+Milestone 87
+------------
+
+  * <insert new release notes here>
+
+* * *
+
 Milestone 86
 ------------