[css-contain-2] Update changelog

Author: frivoal

css-contain-2/Overview.bs

@@ -1990,6 +1990,7 @@ Changes from <a href="https://www.w3.org/TR/2020/WD-css-contain-2-20201216/">202
 * Defined that elements having an ancestor with ''content-visibility: hidden'' don't generate boxes in the top layer
 * Defined that being in the top-layer makes an element relevant to the user
 * Noted that paint effects with non-local effects can limit certain optimization opportunities.
+* Add ContentVisibilityAutoStateChanged event
 
 <h3 id="changes-since-2020-06-03">
 Changes from <a href="https://www.w3.org/TR/2020/WD-css-contain-2-20200603/">2020-06-03 Working Draft</a>