Add notes about changes of cumulative layout shift in M-86

Change-Id: I1286e8e96bd77b27f6d78d36db7e079972e97fce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2360120
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798845}

docs/speed/metrics_changelog/cls.md

@@ -2,6 +2,12 @@
 
 This is a list of changes to [Cumulative Layout Shift](https://web.dev/cls).
 
+* Chrome 86
+  * Fixed bugs about ink overflows (crbug.com/1108622) and transforms (crbug.com/1109053).
+  * Now we aggregate layout shift reports of:
+    * an element and its descendants if they move together, and
+    * inline elements and texts in a block after a shifted text.
+  These changes will affect layout instability score for the specific cases.
 * Chrome 79
   * Metric is elevated to stable; changes in metric definition will be reported in this log.
 * Chrome 77