Add Debugging CLS to speed metrics README.md.

Change-Id: I5196cefa3767f422684a040ec0e6cb9481f90709
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2945370
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#889817}

docs/speed_metrics/README.md

@@ -94,6 +94,7 @@ changelog](../speed/metrics_changelog/README.md) which will be updated over time
   * [Properties of a good metric](../speed/good_toplevel_metrics.md)
   * [Survey of current
     metrics](https://docs.google.com/document/d/1Ww487ZskJ-xBmJGwPO-XPz_QcJvw-kSNffm0nPhVpj8/edit)
+  * [Debugging CLS](http://bit.ly/debug-cls)
 
 ## Talks
   * [Lessons learned from performance monitoring in
@@ -103,4 +104,4 @@ changelog](../speed/metrics_changelog/README.md) which will be updated over time
     by Nicolás Peña Moreno.
   * [Understanding Cumulative Layout
     Shift](https://www.youtube.com/watch?v=zIJuY-JCjqw), by Annie Sullivan and
-    Steve Kobes.
+    Steve Kobes.