[mediaqueries-4] Add changes section

Author: frivoal

mediaqueries-4/Overview.bs

@@ -2154,6 +2154,16 @@ device-aspect-ratio</h3>
 <h2 id="changes" class="no-num">
 Changes</h2>
 
+<h3 id="changes-2020-7">
+Changes since the 21 July 2020 Candidate Recommendation</h3>
+The following changes were made to this specification since the
+<a href="https://www.w3.org/TR/2020/CR-mediaqueries-4-20200721/">21 July 2020 Candidate Recommendation</a>:
+
+* Allow empty functions in <general-enclosed>
+	(see <a href="https://github.com/w3c/csswg-drafts/issues/6803">Issue 6803</a>).
+* Editorial tweak to how the grammar is defined
+	(see <a href="https://github.com/w3c/csswg-drafts/issues/6806">Issue 6806</a>).
+
 <h3 id="changes-2017-9">
 Changes since the 5 September 2017 Candidate Recommendation</h3>
 The following changes were made to this specification since the