Update CHANGELOG.md

Author: brizental

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 [Full changelog](https://github.com/mozilla/glean.js/compare/v0.21.1...main)
 
+* [#796](https://github.com/mozilla/glean.js/pull/796): Support setting the `app_channel` metric.
+  - As described in ["Release channels"](https://mozilla.github.io/glean/book/reference/general/initializing.html?highlight=channel#release-channels).
+
 # v0.21.1 (2021-09-30)
 
 [Full changelog](https://github.com/mozilla/glean.js/compare/v0.21.0...v0.21.1)