Update CHANGELOG.md

Author: mdboom

CHANGELOG.md

@@ -3,6 +3,7 @@
 [Full changelog](https://github.com/mozilla/glean.js/compare/v0.20.0...main)
 
 * [#754](https://github.com/mozilla/glean.js/pull/754): Change target ECMAScript target from 2015 to 2016 when building for Qt.
+* [#779](https://github.com/mozilla/glean.js/pull/779): Add a number of workarounds for the Qt Javascript engine.
 
 # v0.20.0 (2021-09-17)