Add changelog

CHANGELOG.md

@@ -9,6 +9,7 @@
 - Removed `enrollment_id` from being generated or recorded anywhere in the Nimbus SDK ([#5899](https://github.com/mozilla/application-services/pull/5899)).
   - This was originally thought to be of use, but after running the system for sometime, we have found that this isn't needed.
   - In the spirit of reducing unique identifiers in telemetry and in the spirit of Lean Data, we have removed `enrollment_id` (and the code that generates it).
+- Added the feature `activation` event ([#5908](https://github.com/mozilla/application-services/pull/5908)).
 
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