[css-highlight-api-1] Add changelog for updated WD (#8689)

Author: sanketj

css-highlight-api-1/Overview.bs

@@ -626,6 +626,21 @@ Appendix D. Changes</h2>
 
 	<em>This section is non-normative</em>.
 
+<h3 class=no-num" id="changes-since-20211215">
+Changes since the <a href="https://www.w3.org/TR/2020/WD-css-highlight-api-1-20211215/">15 December 2021 Working Draft</a></h3>
+
+	In addition to various editorial improvements and minor tweaks,
+	the main changes are:
+
+	* Specified that highlight repainting has to be done asynchronously.
+		(See <a href="https://github.com/w3c/csswg-drafts/pull/6987">Issue 6987</a>)
+	* Clarify that UAs cannot specify paired default highlight colors for custom highlights.
+		(See <a href="https://github.com/w3c/csswg-drafts/pull/6665">Issue 6665</a>)
+	* Clarify that there is no restriction on custom highlights crossing containment boundaries.
+		(See <a href="https://github.com/w3c/csswg-drafts/issues/4598">Issue 4598</a>)
+	* Added an I18N warning about placement of [=range=] endpoints in the middle of a
+		[=grapheme=] or [=supplementary character=].
+
 <h3 class=no-num" id="changes-since-20201208">
 Changes since the <a href="https://www.w3.org/TR/2020/WD-css-highlight-api-1-20201208/">8 December 2020 Working Draft</a></h3>